Batteries – present and future challenges

Annika Ahlberg Tidblad1, Helena Berg2, Kristina Edström3, Patrik Johansson4, and Aleksandar Matic4

1. Scania CV AB – Materials Technology, Hybrid and Electronics, SE 151 87 Södertälje, Sweden

2. AB Libergreen 3. Department of Chemistry – Ångström Laboratory, Uppsala University,

Box 538, SE 751 21 Uppsala, Sweden 4. Department of Applied Physics, Chalmers University of Technology, SE-

412 96 Göteborg, Sweden

Swedish Hybrid Vehicle Centre 10-2015

Batteries – present and future challenges

In order to find long-term battery solutions for electric vehicles, both the

present and future challenges have to be reviewed. In two complementary

reports, this project sheds light on both aspects by identifying the gaps

between the battery packs and vehicle requirements, and reviewing research

trends of emerging battery technologies in the 2025 perspective.

The first part of the project relates to the current regulations for batteries and

the on-going discussion for the development of future regulations and how this

will influence the present available battery cells and the vehicle requirements.

The gap analysis is based on legislation, scientific publications and vehicle

requirements, both heavy-duty vehicle and passenger car requirements. On-

going research trends are identified to analyse if the gaps can be closed in the

near future. The title for the part is: “White spots on the future battery map

induced ty the development of vehicle regulation”.

The main identified issues are the on-going discussions about the risks with

electrolyte leakage and whether the organic solvents used in the battery cells

could be harmful for battery users in any way. Toxicology shows typical

solvents such as EC, DMC and EMC to be harmful even at low amounts. Beside

the discussion about toxicity, the worry about thermal propagation is a main

concern where relevant test methodology is lacking. The outburst of fire and

explosions are as serious issues as the worry for toxicological effects. The

report suggests future research directions towards solid state batteries or at

least the use of gel type electrolytes to diminish the risk for electrolyte leakage.

The second part of the project; “Emerging Battery Technologies towards 2025”,

reviews research trends to identify how these technologies may fit to different

vehicles and vehicle requirements in terms of performance, weight, volume,

and cost. The main question answered is whether there are any potential post-

Li technologies to replace the Li-ion technology in electric vehicle applications

by 2025. Overall, this study indicates that until 2025, any huge improvements

in the performance of automotive batteries are highly unlikely as there are no

game-changing technologies approaching the consumer market today.

In more detail, higher energy density and higher power capable electrode

materials promise to significantly lower the battery cost by reducing the

amount of material and the number of cells needed for the entire battery pack.

In order to utilise the very attractive energy densities of some of the emerging

technologies, however, very low C-rates must be used. Again, from a pack

perspective, more cells in parallel are then needed to fulfill the performance

requirements. Work is needed to develop new materials and also electrode

couples that offer a significant improvement in energy and power over today’s

technologies.

The cell voltage will also play an important role for the cost: cells having 2 V

lower nominal voltage will result in a battery pack 75% more expensive.

Therefore, the complete battery pack must be evaluated when comparing

emerging battery technologies. As a consequence, cells of lower cell voltage

must be significantly less expensive to produce in order to be competitive at

pack level.

The main research and development needed is related to the next generation

Li-ion batteries operating at high voltage levels (5V). Moreover, cells having

anodes of Si or metallic lithium will be the most attractive solutions for electric

vehicles by 2025 and therefore research should be strengthened for these

concepts. Also, efforts must include the development of novel electrolyte

formulations and additives to form a stable solid electrolyte interphase or even

more efficient solid state electrolytes for improved abuse tolerance, longer life,

low temperature operation, low toxicity and fast charge capability. For power

demanding applications, the most attractive solution by 2025 will be

asymmetrical super capacitors.

Pack-level innovations should focus on technology to reduce the weight and

the cost of thermal management systems, structural and safety components,

and system electronics. Currently, these “non-active” components of a battery

increase the volume, weight, and cost of the finished product. Approaches to

reduce the sizes of these inactive components in the cell and battery should be

pursued. The cost reduction potential is highest for the pack components; a

potential of ca. 75% resulting in a total cost reduction of the battery pack by

about 55% by 2020.

Swedish Electric & Hybrid Vehicle Centre Chalmers University of Technology Hörsalsvägen 11, level 5 SE-412 96 Göteborg Phone: +46 (0) 31 772 10 00 www.hybridfordonscentrum.se

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White spots on the future battery map

induced by the development of vehicle

regulation

Annika Ahlberg Tidblad1 and Kristina Edström2 1. Scania CV AB – Materials Technology, Hybrid and Electronics, SE 151 87

Södertälje, Sweden

2. Department of Chemistry – Ångström Laboratory, Uppsala University, Box 538, SE 751 21 Uppsala, Sweden

Swedish Hybrid Vehicle Centre 10-2015

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Content

1. Introduction

2. Regulatory Landscape

2.1 Battery Directive 2.2 Reach 2.3 UN Recommendation on the Transport of Dangerous Goods –

Model Regulations 2.4 UNECE Vehicle regulations

2.4.1 UNECE R100_02 2.4.2 EVS-GTR 2.4.3 Proposals for future GTR

3. Li-ion batteries (LiBs)

4. Regulatory concerns that will impact the LiB market and future technology developments

4.1 Introduction 4.1.1 Metals in Li-ion batteries 4.1.2 Organic Solvents in Li-ion batteries 4.1.3 Concerns with hazardous chemical substances 4.1.4 Leakage 4.1.5 Venting 4.1.6 Environmental protection – recycling

4.2 Concern with uncontrolled propagation of single cell failure 4.3 Concern with fire hazard 4.4 Concern with performance and durability

5. Research directions as a consequence of the regulations

6. Concluding remarks

Acknowledgements

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Abbreviations BMS Battery Management System C-rate The rate of battery cycling EUROBAT Association of European Automotive and Industrial Battery Manufacturers EU European Union EV Electric Vehicle EVS Electric Vehicle Safety GTR Global Technical Regulation (UNECE 1998 Vehicle Agreement) HEV Hybrid Electic Vehicle LiFePO4 Lithium iron phosphate LiB Lithium-ion battery OEM Original Equipment Manufacturer PAC Protective Action Criteria for chemicals PHEV Plug-in Hybrid Electric Vehicle R Regulation (UNECE 1958 Vehicle Agreement) REACH Registration, Evaluation, Authorization and Restriction of Chemicals REESS Rechargeable Electric Energy Storage System SEI Solid Electrolyte Interphase SIB Sodium Ion Battery UN United Nations UNECE United Nations Economic Commission for Europe WP.29 World Forum for Harmonization of Vehicle Regulation xEV Generic term for electric vehicles including all levels of electric drive

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1. Introduction This is one of two studies of the future needs for batteries with the goal to exemplify what research efforts are needed to reach by 2030. While this report focuses on current legislation as well the ongoing discussions forming the legislation for the future and how this will influence the use of batteries in different kinds of applications, the other report by Helena Berg, Aleksandar Matic and Patrik Johansson elaborates on different possible chemistries and their potential of becoming the leading batteries 2025 in vehicle applications. By separating the study into these two complementary reports we hope to cover the most important future research directions in a 15 year perspective. The total volume of all kinds of batteries that entered the European Union per year are, approximately 800.000 tons of automotive batteries, 190.000 tons of industrial batteries, and 160.000 tons of consumer batteries [1]. These data are from 2009 and all prognostics point at increased levels of battery use. In Figure 1 the global battery market for 2009 is depicted. This study will first give an overview of the regulatory landscape; including existing and near-future legislation applicable for automotive batteries and cells, followed by an overview of the lithium ion battery (LiB) technology, as this is predicted to be the predominant battery technology for electric vehicle (EV) application in a foreseeable future. Finally we discuss foreseeable regulatory impact and the key areas of concern regarding LiB technology from a regulatory standpoint on LiB, focusing on automotive application, and draw up a map of future research directions that can address the existing technology gaps identified by the regulatory landscape.

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Figure 1. Global battery market 2009 [2]

2. Regulatory landscape 2.1 Battery Directive EU legislation is formulated in the Batteries Directive 2006/66/EC [3]. This Directive intends to “contribute to the protection, preservation and improvement of the quality of the environment by minimizing the negative impact of batteries and accumulators and waste batteries and accumulators. It also ensures the smooth functioning of the internal market by harmonizing requirements with regards the placing of batteries and accumulators on the market. With some exceptions, it applies to all batteries and accumulators, independent of their chemical nature, size or design”. In practice this means that the “Directive regulates the marketing of batteries containing some hazardous substances, defines measures to establish schemes aiming at high level of collection and recycling, and fixes targets for collection and recycling activities.” This includes instructions on labeling of batteries and how batteries have to be removed from equipment at disposal. Presently The Battery Directive restricts content of mercury (<5 ppm) and cadmium (<20 ppm) in portable batteries. There are exemptions from these limits for specific applications but these are reviewed on a regular basis to determine if the exemption should be revoked due to technology advances on the market. Marking requirements apply to batteries containing higher concentrations of mercury and cadmium than the specified limits as well as batteries containing <40 ppm lead. The collection and recycling targets are progressive to stimulate the market to continually develop more efficient processes.

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The Battery Directive also aims to improve the environmental performance of all operators involved in the life cycle of batteries and accumulators, e.g. producers, distributors and end-users and, in particular, those operators directly involved in the treatment and recycling of waste batteries and accumulators. Producers of batteries and accumulators and producers of other products incorporating a battery or accumulator are given responsibility for the waste management of batteries and accumulators that they place on the market. This description of legislation comes from EUROBAT, an industry organization promoting the interests of the European industrial, automotive and special battery manufacturers within EU [4]. The mail goal for the EU Battery Directive is to minimize the negative impact of batteries on the environment and improve their overall environmental performance. Technical development towards improving the environmental performance of batteries is encouraged. However, in recognition of the chemical nature of batteries and ingoing substances, all batteries should be collected. The Directive also ensures the Member states to take care of storage and recycling of spent batteries in an appropriate way which is fit for the purpose. The progress of the areas covered by the Directive has to be reported regularly. In this Directive automotive batteries are defined as SLI (starting, lighting, ignition) whereas hybrid and electric vehicle traction batteries are treated as industrial batteries. The Battery Directive acts as a framework law, foreseeing further legislation in the field of batteries.[3] The fast development of new battery technologies, e.g. the vast family of rechargeable lithium batteries that currently find their ways into more and more applications, give at hand that a more thorough review and deliberation of what substances may need regulation is expected in a foreseeable future. The lithium based chemistries do not contain materials where mercury, cadmium or lead are predominant, nor common contaminants. However, there are multiple other substances contained in these batteries with the potential to cause adverse effects on the environment. There are also secondary legislation on batteries formulated after 2006. Here is a list of what they contain: Regulations about the calculation of recycling efficiencies of the recycling process of waste batteries and accumulators (Commission Regulation (EU) No 493/2012 of 11 June 2012) [5]; This regulation states that the target for collection rates should be at least 45% by September 26, 2016. The targets are

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defined in terms of average weight. The target is 50% for all other batteries than lead-acid (target 65%) and nickel-cadmium (75%). . Rules regarding the capacity labeling of portable secondary (rechargeable) and automotive batteries and accumulators (Commission Regulation (EU) No 1103/2010); [6] A questionnaire for the Member States reports on the implementation of the Directive 2006/66/EC (Commission Decision 2009/851/EC); [7] Requirements for registration of producers of batteries and accumulators in accordance to the Directive 2006/66/EC of European Parliament and of the Council (Commission Decision 2009/603/EC); [8] There are, however, not only legislation for environmental and safety concerns but also regulations about the market of batteries. Some examples are given here: A common methodology for the calculation of the annual sales of portable batteries and accumulators to end-users (Commission Decision 2008/763/EC); [9] How to place batteries and accumulators on the market (Directive 2008/103/EC [10] of the European Parliament and of the Council of 19 November 2008 amending Directive 2006/66/EC on batteries and accumulators); [3] Implementing power conferred on the Commission (Directive 2008/12/EC of the European Parliament and of the Council of 11 March 2008 [11] amending Directive 2006/66/EC on batteries and accumulators and waste batteries and accumulators); [3] In addition to adopting the above legislation, the Battery Directive sets the following deadlines for implementation:

26/09/2009: article 12 requirement for producers or third parties to set up schemes for treating and recycling waste batteries and for all collected batteries to undergo treatment

o 26/11/2011: Recycling of batteries must meet the recycling efficiencies listed in Annex III Part B

26/09/2009: Capacity marking rules must be followed (article 21) 26/09/2010: Review of nickel cadmium exemption for cordless power

tools. Due to the advancement of, for example rechargeable LiB technologies, the exemption for use of nickel cadmium batteries in

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cordless power tools will be revoked for products placed on the market after 2015.

26/03/2010: Calculate recycling efficiency 5th full calendar year after entry into force: 1st calculation of collection

rates by Member States 2012: Collection Rate of 25% of portable batteries 2016: Collection Rate of 45% of portable batteries

o Member States must report to commission every year (before end June) on progress of collection

26/09/2012: First national implementation report (article 22), final due date 26 June 2013

2015: Commission must review the implementation and impact of the Directive (article 23)

2.2 REACH [12,13] REACH stands for Registration, Evaluation, Authorization and Restriction of Chemicals. It is a European Union regulation dated from 18 December 2006. REACH regulates what kinds of substances that can be used in products on the European markets. The regulation restricts the use of some substances that that are currently used as active materials in industrial and automotive batteries and accumulators, but the list also includes substances that may be included in future battery technology products. The substances are collected in Article 58 § 1 (e) and Article 58 § 2 of Regulation (EC) No 1907/2006 (REACH). 2.3 UN Recommendation on the Transport of Dangerous Goods – Model Regulations All the regulatory activities described above are focused on environmental

protection and waste stream control comprising all different kinds of batteries

and accumulators, regardless of application. However, with the growing market

of primary and secondary lithium batteries, there is an increasing concern from

the industry and the authorities aimed at safety related issues. Transportation

and handling of lithium batteries during transportation is one aspect that

surfaced due to repeated incidents involving consignments of lithium batteries

during transportation. International regulation for all modes of transport, land,

sea and air, are based on the UN Recommendations on the Transport of

Dangerous Goods – Model Regulations (ST/SG/AC.10/1/Rev.18) [14]. The

Model Regulations are translated into transportation legislation comprising

detailed instructions for packing, marking, documentation and testing based on

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mode of transport: ADR/RID for land transport (road and rail, respectively),

IMDG Code for sea transport and IATA/ICAO for air transport. Except for the

IATA/ICAO, which is revised on a yearly basis, the other regulations are revised

every other year. The packaging instructions are based on how the cell or

battery is transported, which is indicated by the UN identity code:

UN3090 Lithium primary cells and batteries (incl. Li metal and Li alloy)

UN3091 Lithium primary cells and batteries in or with equipment

UN3480 Lithium ion cells and batteries (incl. Li-ion polymer)

UN3481 Lithium ion cells and batteries in or with equipment

UN3171 Battery propelled vehicle

There are different packing instructions for cells/batteries depending if they are

new, damaged/defective or waste for disposal/recycling.

All primary and secondary lithium cells and batteries must be tested and

approved against UN 38.3 (ST/SG/AC.10/11/Rev.5) [15]. Testing is performed

hierarchically, so that cells are tested first, and, thereafter, any cell assembly

subjected to transportation must also be tested and found compliant. The tests

are representative of conditions that can occur during normal transport and

handling:

T1: Altitude simulation

T2: Thermal test

T3: Vibration

T4: Shock

T5: External short circuit

T6: Impact/crush

T7: Overcharge

T8: Forced discharge

All legislation mentioned so far, is generally applicable and not specific for

vehicle application.

2.4 UNECE Vehicle regulations

As with transport regulation, which is global, international vehicle regulation is

also handled under the UNECE umbrella [16]. There are two different

categories of international vehicle regulation: the “R” regulations that are

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developed under the 1958 Vehicle Regulation Agreement and the “GTR”

regulations that are developed under the 1998 Vehicle Regulation Agreement.

The geographic applicability of the “R” and the “GTR” regulations, respectively,

depends on the contracting parties, i.e. countries, that have signed the

respective agreements. The two regulatory systems are related in the sense

that they affect each other. When there is both an “R” and a “GTR” regulation

on the same technology area, they have to harmonize and not imply

contradictory requirements. Additionally, regulations should as far as possible

be technology agnostic and not impose undue design restrictions. The benefit

of having global regulations is of course that it replaces national regulations in

the countries of the contracting parties, thus removing market barriers for

OEMs by introducing the same technical requirements on multiple markets. “R”

and “GTR” regulations are developed through collaboration between

authorities and market stakeholders, e.g. vehicle manufacturers, suppliers to

the vehicle industry and Testing Services.

The introduction of electrified vehicles, xEV (HEV, PHEV, BEV), using LiB on the

market has raised concern about safety performance of the rechargeable

electric energy storage system (REESS). The reason is the high energy density of

the LiB, both in terms of gravimetric and volumetric energy capacity, and the

history of LiB related incidents that have occurred in other product segments,

notably consumer products (laptops and cell phones) as well as severe

incidents with LiB during transportation, resulting in significant material

damage and/or personal injury.

2.4.1 UNECE R100_02

REESS safety performance is regulated by UNECE R100_02 (1958 Vehicle

Agreement) [17]. This regulation is applicable within EU, Japan, Korea and a

diversity of countries in Asia and Africa and contains requirements for electrical

safety of xEV, independent of the battery technology used in the REESS. It is

very challenging to develop requirements that are applicable across battery

chemistries and technology solutions. R100_02 [18] replaces R100_01 [19] and

is currently voluntary for vehicle type approval, but becomes mandatory for

vehicles seeking type approval from July 2016. The scope of the regulation is

safety of the high voltage traction battery system in use, that means during

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normal operation. The tests are designed to evaluate typical environmental

conditions and potential hazardous scenarios that can occur during normal use

of the vehicle:

Vibration

Thermal shock and cycling

Mechanical impact

Mechanical integrity

Short term ground fire exposure

External short circuit

Overcharge protection

Overdischarge protection

Overtemperature protection

R100_02 applies to both passenger vehicles and heavy duty vehicles, but

includes some exemptions from specific requirements for heavy duty vehicles

due to technical differences as well as fundamental differences in the overall

regulatory practice for passenger vehicles vs. heavy vehicles.

2.4.2 EVS-GTR

An EVS-GTR for improved vehicle safety is currently in progress with USA,

China, EU, Japan and Korea as co-sponsors for the regulation, Figure 2. A final

draft is to be submitted to UN ECE for vote at the end of 2015, however it is

expected to be delayed by at least 1 year. The scope of the EVS-GTR has been

expanded compared to R100_02 in that the GTR also considers charging and

post-crash scenarios. Initially, the EVS-GTR was intended for passenger vehicles

only, but applicability to heavy duty vehicles is being considered. Since there

are some fundamental differences in technology solutions between heavy duty

vehicles and passenger vehicles, such as charging strategies and crash

management, inclusion of heavy duty vehicles in a passenger vehicle regulation

is not straight forward and requires a number of difficult technical

deliberations.

Test proposals considered for the EVS-GTR include the in-use tests from

R100_02, sometimes with modifications and additional requirements, plus a

number of additional tests, for example:

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Water protection test

Long term external fire exposure

Protection against unintentionsal exposure of humans to high voltage

post-crash

Protection against uncontrolled thermal propagation

Additionally, the EVS-GTR is considering a number of new acceptance criteria

related to emissions of hazardous chemical substances and thermal events.

Figure 2 The makings of and Electrical Vehicle Safety Global Technical

Regulations (EVS-GTR). The GTR merges the national regulations of the co-

sponsors with existing UN ECE regulations and new requirements for electric

vehicle safety.

2.4.3 Proposals for future GTR

During 2015, UNECE WP.29 has initiated prestudies to investigate the need for

and feasibility of developing new GTRs for xEV on the following topics,

suggested by different countries:

Battery Performance and Durability (USA and Canada)

xEV battery recycling

Determining power of EVs (Germany and Korea)

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Method of stating energy consumption of xEVs (China)

If the prestudies support development of GTR regulation on any of the above

topic, these activities are projected to will in 2016.

3. Li-ion Batteries (LiBs)

To better understand why all the above described regulations and discussions about new legal measures are so concerned with the cell chemistries we will give a short description of what a LiB is, its cell contents and principally how the battery works. The LiB is a whole family of different possible materials that can be selected according to where the battery will be used [20, 21, 22, 23]. There is also a complementary group of lithium-based batteries considered as future possibilities – known as “beyond lithium” – such as the lithium-sulfur and lithium-oxygen batteries [24, 25, 26, 27, 28, 29]. They are not going to be considered here since they are too far away from the market. However, if they would enter the market it will be the same regulations that handle their safety, transportation and market aspects as for LiBs. Another more realistic (from a market perspective) beyond lithium chemistry is the sodium ion battery (SiB) [30, 31, 32]. A SiB is based on the same principle of intercalation/insertion electrodes as LiBs but with the mobile ion being the larger sodium ion making the choices of electrode and electrolyte materials vast but not as large as for the smaller lithium ion in LiBs. In general a battery converts chemically stored energy to electrical. In a LiB the stored energy can be calculated in how much lithium that can be hosted in the atomic structures of cathode and anode materials. This is described as the lithium ions being intercalated or inserted into the electrodes. It is the cathode which is the bottle neck for how much total energy (how much lithium that can be inserted) that a LiB can deliver. There is therefore a constant hunt for better cathode materials. A commercial LiB consists most often of a graphite negative electrode, a positive electrode of a transition metal oxide or a phosphate and an electrolyte composed of an organic solvent containing a lithium salt (or a mixture of lithium salts) and additives to enhance stability and safety. The positive and negative electrodes are not short circuited due to the electrolyte which is soaked into a porous polymer separator between the electrodes, which allows the transport of ionic charge carriers but prevents electrical contact. An

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example of a typical LiB is shown in Figure 3. When the battery is made it is in a discharged state. The battery must start by being charged which means that the lithium ions must be transported through the electrolyte and intercalated into the graphite electrode. The battery is now ready to be used. If the cathode material is an oxide the potential of the battery is larger than if the cathode is lithium iron phosphate. During the first charge there is also a reaction with the (at these potentials close to that of lithium metal) electrolyte taking place on the graphite surface. A so called SEI (Solid Electrolyte Interphase) is formed. This SEI is about 20 nm thick and protects the graphite from being destroyed during the battery cycling. The role of this SEI for battery safety and the battery lifetime is extensively discussed [33, 34, 35, 36, 37].

Figure 3. Schematic diagram of a LiB composed of intercalation electrodes. Graphite is to the left and an insertion cathode to the right.

Most LiBs operate in a temperature window of -15°C to + 60C. An interesting exception is the Bolloré Blue car found on the streets of Paris where the Li-battery has a metallic lithium negative electrode, a polymer electrolyte,

LiFePO4 as the positive electrode and operates at +80 C. One of the limitations is the low ionic and electric conductivity in the battery components. This will limit how fast a battery can be charged. Due to its

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operational principles, LiBs are hence not as fast in building up charge as, for example, supercapacitors. There are several text books on LiBs that give details of the chemistry taking place at cell level. One recent with focus on automotives has been authored by Helena Berg [38].

Figure 4. Energy vs. Power for different battery and capacitor systems [39]. The Ragone plot in Figure 4 shows that the energy density of a LiB is still not high enough to meet the needs for EVs while both power and energy density already meet the requirements for Plug-in Hybrid Electric Vehicles (PHEV).

4. Regulatory concerns that will impact the LiB market and future technology developments

4.1 Introduction In this report we only discuss substances that are included in LiBs and in batteries beyond LiBs included in Article 58 § 1 (e) and Article 58 § 2 of REACH. All existing battery technologies contain some unwanted substances, from environmental and toxic considerations. These substances may be the substances making up the active materials in the electrochemical cell, in which case their presence is purposeful, and therefore it can be difficult to phase out these substances without affecting the energy storage properties or electrical

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performance of the battery. In other cases, the hazardous substances are introduced into the cell as contaminants to the main constituents. In this case, raw material sourcing and production process parameters are the main determinants for the quantity of these substances in the cell. Removing trace contaminants of heavy metal, for example, may not have any detrimental effect on the performance of the battery, but the technical challenges or costs involved may be prohibiting. Traditional, mercury, cadmium and lead have been considered as the most problematic heavy metals in batteries, which is why these substances are regulated by the Battery Directive [3]. Technology advances have made it possible to almost completely eliminate the amounts of mercury, cadmium and lead in alkaline primary and rechargeable batteries. However, these metals are not the primary environmental concern in LiB. These batteries contain other types of electrode materials as well as organic solvent electrolytes with highly reactive salts, PF6 is currently the prevalent conductive salt in LiB on the market. Examples of heavy metals that occur in LiB include nickel, cobalt, copper, chromium, vanadium and a vast amount of different species at very low concentration, for example thallium and iridium. With the introduction of large scale LiBs on the market, in xEVs and in various types of large scale stationary applications, there has been an increasing focus on the potential hazards of the organic solvents and salts that can be found in the electrolyte. How toxic are they? What happens if a battery cell or battery pack starts to leak electrolyte or if the battery gets pressurized and starts venting electrolyte fumes into the surrounding? What substances can be expected to be present and what is the range of vapor pressures expected in commercially available cells? How will the substances spread from the source and what is a realistic exposure hazard for occupants inside the vehicle as well as for people residing outside of the vehicle? If the electrolyte fumes ignite and start to burn, what chemical species are expected in the fire smoke and what factors influence formation of HF, CO, polyaromatic carbons and other toxic substances under these conditions? Since the exact electrolyte composition is a well-guarded secret by the cell manufacturers, how can generic safety regulations be developed that can handle the wide variety of LiBs presently on the market as well as those expected within a 10-15 year time-frame?

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4.1.1 Metals in Li-ion batteries Historically, the only metals of concern for batteries have been mercury, cadmium and lead. With the introduction of LiB, this map is changing. The metals and metallic contaminants relevant for LiB are different, but not necessarily less problematic from an environmental point of view. Additionally, the wide variety of possible cell chemistries of LiB compared to lead-acid and rechargeable alkaline battery technologies imply that efficient recycling processes are more difficult to develop. The first task that needs to be done is to determine which, if any, metals that need to be regulated. The next step is to determine which, if any substances that needs to be restricted, what would constitute acceptable levels as well as universally suitable methods to detect and quantify the presence of the restricted metals.

4.1.2 Organic solvents in Li-ion batteries Electrolyte solvent chemistry in LiB is a complex area of study. Whereas there are a limited number of common organic solvents making up the bulk of the electrolyte, typically carbonates such as ethylene carbonate and dimethyl carbonate, there are multiple additional constituents present in the electrolyte in order to achieve the required performance of the particular cell chemistry and design. These additives can be film forming to prevent corrosion reactions between the electrolyte and electrode surface. They can be molecules trapping the anion from decomposing during the cycling or they can be flame retardents for improved safety or redox shuttles to prevent over-charging or over-discharging of the battery. Some examples of efficient additives are described in ref. 40, 41 and 42. There are also examples of solid electrolytes such as polymer electrolytes [43, 44] and lithium-conducting ceramics [45, 46]. These are yet less commonly used due to their lower ionic conductivities and in some case the compounds are brittle. In this respect it is also important to mention ionic liquids that might be an alternative due to their high thermal stabilities but still cost is an issue and another drawback is a lower ionic conductivity [47].

4.1.3 Concern with hazardous chemical substances From a regulatory perspective, concern is focused on potential toxic effects of electrolyte constituents and reaction products leaking or venting out of the battery into the environment. The toxicology assessment of substances

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reported to be used as electrolyte solvents and additives is still at an early stage and the tendency from the authorities is to be extremely cautious about allowing scenarios where the vehicle occupants and general public may be exposed. Present day LiB electrolytes are complex mixtures of solvents, conductive salt and various additives. The recipes are well-guarded secrets and it is nearly impossible to gain detailed knowledge of all constituents and their concentrations in the solution. The high flammability and propensity to generate fire upon release at elevated temperatures is another reason for concern. The decomposition reactions of the electrolyte is oxygen forming, which further increases fire hazards and make any fires challenging to extinguish, especially if the cathode material is an oxide which will add fuel to the fires. In order to overcome the concerns, electrolyte research is essential. There are two possible routes to satisfy the immediate concerns: solid electrolytes that contain no liquid components capable of escaping from the cell and liquid electrolytes made from substances that are well documented as non-toxic, non-volatile and not flammable. Additive research aimed at solving existing performance and durability issues would further complicate the electrolyte chemistry and there is often very little information about what additives are present or the amounts added. Hence, although new additives may benefit the cell and battery performance, this line of research runs a risk of not meeting the concerns expressed by regulating authorities about potential health effects to vehicle users, first and second responders or the general public surrounding a vehicle where there is a battery incident involving release of electrolyte from the cells. In the regulatory context, electrolyte release is defined by two separate processes: leakage and venting, depending on the origin and nature of release.

4.1.4 Leakage Leakage is generally considered to be electrolyte release in liquid form, for example as a result of tab weld failure or if there is a crack or rupture on the battery cell casing or in the cell venting devise through which electrolyte can seep out. The organic solvents currently used in LiB electrolytes are volatile and flammable, the potential occurrence of leakage is causing concern. One of the risks considered is that of igniting the escaped electrolyte and thereby releasing enough heat energy by combustion to initiate a thermal event that may propagate into a more serious incident. The second hazard is that of exposure of the vehicle occupants to toxic substances. Currently, there are differing opinions about the actual toxic effect caused by both acute and long term

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exposure to electrolytes and electrolyte fumes, and there is a risk of assuming an extremely precautionary approach to when making toxicological assessments in order to avoid adverse effects on the vehicle occupants as well as road rescue workers and first responders. However, overestimating toxicity resulting in very restrictive requirements on automotive battery electrolytes may jeopardize the future of LiB in xEV applications and it should be noted that there are no known reported field incidents of harmful situations involving electrolyte leakage or venting affecting vehicle occupants. The realistic toxic hazard situation depends on the volume and concentration of the species released as well as the actual exposure parameters including the duration of exposure and the effective uptake of the chemical species by the human body. The likelihood of exposure is another parameter that needs to be considered as well as a fair comparison with the risks posed by competing propulsion technologies. LiB are generally considered leakage free as they belong to a sealed battery technology, designed to have no exchange of materials with the environment during normal operating conditions. Leakage will, under this assumption only happen as a consequence of a severe cell or battery failure, and it is therefore considered very unlikely to happen. Additionally, electrolyte is a costly cell component and therefore the amount of electrolyte fill into the cells is precisely dimensioned to provide sufficient wetting of the electroactive materials. Hence, aside from the initial charge-discharge cycles, there is no free electrolyte present inside the cell. The reason that small amounts of excess electrolyte can be observed when opening fresh cells is that electrolyte absorption into the materials is a stepwise process which also involves formation and stabilization of the SEI layers on the electrodes. The possible initial overfill condition is intended to prevent drying out inside the cell leading to premature ageing; irreversible capacity and power loss. The amounts of overfill is proportional to the cell size, but is limited to the milliliter range for current automotive cells on the market, and partly reflects the overall engineering quality of the manufacturer. Premium brand cells typically require less overfill than more basic LiB cells. Regulatory efforts targeting controlling any effects of electrolyte leakage may become unproportionally stringent and limiting for the technology. The risk assessment needs to be balanced in terms of predictions of realistic likelihood of leakage and a realistic approach to assess the potential toxicological effects on humans. Advances in electrolyte and cell research that reduces the quantity of electrolyte needed in order to wet the active materials and ensure long

21

operational lifetimes is one possible approach to addressing the concerns. Another approach is to look for other types of functional electrolyte systems that can replace the organic solutions, e.g. solid electrolytes or other types of conductive liquids that are compatible with the electroactive materials on the electrodes and stable at the required cell voltages.

4.1.5 Venting Venting is an event associated with electrolyte leakage, but which also implies that additional chemical processes are occurring inside the cell causing build up of gaseous constituents from electrolyte and electrode decomposition reactions. If this happens to the extent that there is significant gaseous evolution, LiB are designed to vent in a controlled manner to prevent further more severe situations from occurring, e.g. explosion. When the cell vents, several species are released including but not limited to, CO2, CO, H2, O2, hydrocarbons and HF. Figure 5 shows typical gas constituents formed during excessive heating and combustion of common LiB electrolytes. The gas also contains a fine aerosol of electrolyte solution. The relative amounts of the various constituents depend on the conditions in the cell prior to and at the time of venting. If the fumes should catch fire, it is nearly impossible to extinguish due to the formation of oxygen as a reaction product, enabling combustion even when external oxygen sources are eliminated.

The venting fumes are very hot and constitute a risk of attaining burns in addition to potential toxic effects. The general strategy for managing this risk is to ensure that the vehicle design does not allow venting fumes to enter into the passenger compartments of the car. There have been proposals to enforce strict control of venting gas, such as on-board gas monitoring and a very restrictive view on venting as an acceptable failure mode for cells. However, this may drive cell manufacturers to construct cells that vent at higher temperatures and pressures, thus leading to increased risk of severe incidents from other failure paths.

22

Figure 5 Gaseous species formed on extreme heating and combustion of a

typical Li ion battery electrolyte mixture [48].

4.1.6 Environmental protection – recycling

The research trend towards inexpensive and common materials in LiB to bring down the total costs of the energy storage system in automotives creates an added challenge in itself. A primary driver for recycling is that there is a secondary market willing to buy the recycled material. If the economic incentive is not naturally in place in the market, then the entire cost of recycling needs to be financed by other sources.

4.2 Concern with uncontrolled propagation of single cell failure Following the reports of spectacular LiB failures where a thermal event has cascaded and propagated from a single cell event into a multi-cell or even a whole battery thermal runaway, there is concern of the consequences if this was to happen to a xEV battery. For one, the automotive LiBs comprise more cells and have higher voltage and power specifications than other LiBs that the general public comes into contact with. This is regarded as a potential risk of causing human and material injury. Second, the automotive industry typically mitigate the risk by robust system engineering and design including strategies for handling potential risk situations. However, from a regulatory standpoint it

23

is desirable to impose a test with acceptance criteria, and this is not an easy task, considering the variety of design solutions developed by different manufacturers, who are reluctant to share detailed information about proprietary system safety and performance features. This is a source of controversy between the industry and the regulating bodies, as it is challenging to develop a test procedure that does not require manipulation or modification of the REESS in such a way that the safety design features that the manufacturers have put in place to mitigate risks are not compromised. Although propagation is a possibility, especially in the event of an internal short circuit inside a cell in the pack, the likelihood for occurrence has been deemed low by the automotive industry. The BMS structure of the automotive REESS limits normal operations well within the defined voltage, current and temperature limits of the cell chemistry. The position is that sufficient safety is ensured provided that the system can contain any incident and prevent it from spreading beyond the LiB. Traditional ways of assessing internal short circuits on cell level have proven to be far too aggressive to be representative of a field event in a xEV pack. Various thermal propagation initiating methods have been considered without success in finding a viable solution to this problem. These include:

1. Nail penetration 2. Nail prick (incomplete penetration) 3. Blunt nail crushing 4. Inserting an external heater into the pack 5. Overcharge 6. Inserting defined “contaminating” Ni particle into a cell 7. Local chemical explosive 8. Extended external fire exposure

The first 6 methods are known from academic research and from cell and battery safety standards development, but they are not suitable for regulatory safety assessment. Standards are often used to assess general battery behavior and investigate the limits of a technology. The acceptance criteria for the test are defined by the parties performing the test based on the purpose of the investigation. Regulatory acceptance criteria must be fulfilled in order for a vehicle to be approved for the market. If the assessment method accelerates the risk scenario too much, then there is a danger of excluding viable vehicle designs from the market based on flawed test methodology. Hence, the equivalency to the envisioned real life event must be ensured. None of the

24

methods listed above meet this requirement. In all cases, there is an acceleration of the triggering event beyond that of what is expected in the case of a spontaneous internal short circuit field failure triggered in a cell. For example, in several cases, it is difficult to limit the trigger to a single cell. Electrical heaters and external heat sources apply heat for too long to be representative of an internal single-cell thermal failure. A majority of the methods also involve manipulating the REESS physically and/or disabling necessary safety prevention features intentionally designed into the battery in order to mitigate thermal events. This is unacceptable as it is likely to affect the outcome of the test and poses questions about the equivalency of the test result to real life conditions. A preferable LiB development solution would, of course, be realizing a cell design and chemistry that is immune to propagating thermal events. However, research towards more reliable, balanced and realistic test methods is also necessary.

4.3 Concern with fire hazard As mentioned earlier, the currently used electrolytes in LiB are highly flammable and imply a risk of fire. The ignition temperatures are fairly low

(~450-470 C). During venting, electrolyte ejection at high pressure may result in flames extending far from the battery or vehicle. Fires are documented to be difficult to extinguish due to the self-sustaining ability to provide oxygen for combustion by cell internal reactions involving decomposition of the electrolyte.

4.4 Concern with pPerformance and dDurability The most recent regulatory propositions include methods for setting requirements on performance and durability of the battery system. The basis for this line of regulatory activities is environmental protection. The large scale LiB systems used in vehicles would impact greatly on the quantity of waste batteries in case the realized life expectancy does not reach the expected life time of the vehicle, which would mean introducing vast quantities of heavy metals and other substances into existing battery waste streams. The other part of the regulation proposal is to look into methods of determining the efficiency of the electrical power system including:

25

1. Method of stating energy consumption 2. Determining power of EVs

The challenge for the automotive industry is the dependence of performance and durability on usage patterns and uncontrolled external parameters as well as the design of the battery system. Performance in this sense has traditionally been viewed as a concern between the automotive makers and their clients, handled by warranties and other market drivers. However, the introduction of new vehicle technology seems to stimulate rule making authorities to investigate new approaches to apply the mandate of regulation. One possible explanation may be the global concern in the market about the actual life time of LiB REESS in vehicles. The technical specifications indicate that the original REESS should last the entire life time of the vehicle, but if this life time expectancy is possible to reach with existing battery technology is still debated and has yet to be proven from normal operational use. Regulation can be considered as a means of transferring the “risks” of not meeting targeted performance from the users to the manufacturers and forcing the industry to prioritize the development of efficient and durable battery solutions for future vehicle applications.

5. Research directions addressing regulatory concerns There are several aspects ranging from LiB cell content to a battery pack of LiB cells that have to be taken into account when describing future research directions. We base our suggestions mainly on chapter 4 and the environmental and hazardous discussions which form the regulatory considerations today and what it seems in the future. We see several important topics to consider:

1) Research towards more reliable, balanced and realistic test methods Test methods at battery pack level are performed and developed by companies. However, a better scientific description how different ways of testing batteries will impact the life time and safety of a battery is still lacking. Methods for testing the safety of cells not only when they are freshly produced need to be developed. Most battery cell failures take place after some hundred cycles rather at their pristine state. Additionally, methods to assess safety on a system level are required since single cell test results are not representative of the REESS system or vehicle safety performance. This should not be surprising, since the battery pack and REESS system contain several layers of safety

26

functionality not available on cell level. There are also indication that size itself plays a part in the test outcome, for example, fire testing of single cells and cell assemblies comprising different amounts of cells [49] indicate that heat release rates of single cells are much higher than for multi-cell test objects. This seems to be due to shielding effects as well as the greater mass of the multi-cell units. The system level tests have to be non-intrusive so as to eliminate the risk of flawed test results due to manipulated test objects and not real safety failures of the system.

2) Research towards more stable SEI on high capacity anodes

The main environmental issues regarding anode materials used today, which is mainly a carbon (graphite) material and a cellulose or polymer-based binder, is the energy needed for highly purifying the graphite and the large amount of water necessary for electrode fabrication. The safety issues are related to the SEI formed during cycling of a LiB. This SEI is formed due to thermodynamically instable organic solvents but it protects the graphite from unwanted side reactions at the same time as it consumes lithium during its formation. When heating a LiB cell the first reaction will be that the SEI will react by emitting some heat. It is not a large amount but could be large enough for heating the electrolyte and then the cathode material. Normally there are protections within the cell that can take care of the heat generated. It can be a separator that melts and shuts down the cell or other ways of handling this issue. On a REESS level, there is active or passive cooling in place, regulated by the BMS, that kicks in and lowers the temperature even before critical temperatures are reached. The BMS will also limit the current flow through the battery pack to control further thermal evolution or, in the event of a severe temperature increase, disconnect and disable the battery. There are several research directions regarding the negative electrode. One is to incorporate silicon in the graphite to increase the amount of storage. Another important direction is to stabilize the SEI and increase its temperature stability. This can be done by effective thin coatings or by the use of solid electrolytes.

3) Research towards more stable high capacity cathodes One reason for using lithium iron phosphate instead of transition metal oxide cathodes is the thermal stability. The trend is, however, to increase the volumetric capacity of the total LiB cell and then a high capacity cathode is a

27

necessary route to take. This implicitly means that some oxide must be used. During a thermal runaway the oxygen in the oxide will add extra fuel to the fire. If a high capacity cathode will be used, the research direction for the full cell development should be to make the whole cell more thermally stable by improved cell design and novel electrolytes. A solid electrolyte with high thermal non-leaking properties will be beneficial also for a highly reactive electrode material.

4) Electrolytes As discussed earlier in this report the research on additives such as film formers, flame retardants and redox shuttles will not be enough to stabilize a liquid organic solvent electrolyte. The future research trends will in this respect be a turn back to solid electrolytes such as polymers or even ceramic inorganic ionically conducting membranes. If the battery cells could be allowed to operate at slightly higher temperatures than room temperature then the drawbacks of low conductivities could be hampered. New deposition techniques for inorganic membranes are also supporting the development of thinner ceramic membranes and plasticizers that can make them less brittle. We forsee that this probably will be the most important research directions the coming years to hinder a negative development for LiBs in terms of regulatory constraints due to fear of leakage and venting of battery cells. Inherently non-toxic and non-flammable liquid electrolytes would also meet the requirements of the market. However, this would most likely mean needing to move away from the present organic solvents and conductive salt solutions. Research directions involving ionic liquids are ongoing, and may turn out to be a viable electrolyte choice from a safety standpoint. However, as with solid electrolytes, ionic liquids typically require higher operating temperatures in order to achieve reasonable conductivity.

6. Concluding remarks This report has had the aim to discuss the current regulatory developments that influence the future market and use of LiBs for automotives. Based on the ongoing discussion about the environmental and health issues concerning liquid electrolytes used today we see that important research directions are to make larger efforts in finding thermally stable solid non-brittle electrolytes with high ionic conductivities for LIBs to be undisputable as energy storage solutions for the automotives of the future.

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Acknowledgements The support of director Elna Holmberg and the funding of SHC for this work is highly appreciated and acknowledged. The Swedish Energy Agency is also acknowledged. References [1] http://ec.europa.eu/environment/waste/batteries/pdf/exten_impact_ assessment.pdf 2015-06-19 [2] Battery University from Frost and Sullivan 2009. [3] DIRECTIVE 2006/66/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 6 September 2006 on batteries and accumulators and waste batteries and accumulators and repealing Directive 91/157/EEC (Text with EEA relevance) [4] EUROBAT, www.eurobat.org 2015-06-19 [5] http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32012R0493. 2015-06-19 [6] http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2010:313:0003:0007:EN:PDF [7] http://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:32009D0851 [8] http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2009:206:0013:0015:EN:PDF [9] http://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:32008D0763 [10] http://eur-lex.europa.eu/legal-content/SV/ALL/?uri=CELEX:32008D0763 [11] http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2008:076:0039:0040:EN:PDF [12] https://www.kemi.se/sv/Start/Reach-forordningen/ [13] http://www.reach-compliance.eu/web/english/services/european_union/REACH/registration/REACH-registration-services.html?gclid=CjwKEAjwnpSsBRDH3pT2-7q55R4SJABRiNyTyLarlSnxPkfoIoik3zw6GCnwV-uWe2tEiOyb6n3QLhoCXsPw_wcB [14] http://www.unece.org/?id=3598 [15] http://www.unece.org/fileadmin/DAM/trans/danger/publi/manual/Rev5/English/ST-SG-AC10-11-Rev5-EN.pdf [16] http://www.unece.org/trans/main/welcwp29.html [17] http://www.unece.org/trans/main/wp29/wp29regs81-100.html [18] http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/2013/R100r2e.pdf [19] http://www.unece.org/fileadmin/DAM/trans/doc/2010/wp29/ECE-TRANS-WP29-2010-52e.pdf [20] J.-M Tarascon and M. Armand, Nature 414 (2001) 359-367

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Swedish Electric & Hybrid Vehicle Centre Chalmers University of Technology Hörsalsvägen 11, level 5 SE-412 96 Göteborg Phone: +46 (0) 31 772 10 00 www.hybridfordonscentrum.se

           

     

Emerging  battery  technologies  towards  2025    

   

                                   

Helena  Berg,  AB  Libergreen  Aleksandar  Matic,  Chalmers  Patrik  Johansson,  Chalmers  

 Göteborg,  May  2015.  

 

2  

Executive  Summary    Higher   energy   and   higher   power   capable   electrode  materials   promise   to   significantly  lower   the   battery   cost   by   reducing   the   amount   of   material   and   the   number   of   cells  needed  for  the  entire  battery  pack.  The  cell  voltage  will  also  play  an  important  role  for  the  cost:  cells  having  2  V  lower  nominal  voltage  will  result  in  a  battery  pack  75%  more  expensive.   Therefore,   the   complete   battery   pack   must   be   evaluated   when   comparing  emerging   battery   technologies.   As   a   consequence,   cells   of   lower   cell   voltage  must   be  significantly  less  expensive  to  produce  in  order  to  be  competitive  at  pack  level.      In   order   to   utilise   the   very   attractive   energy   densities   of   some   of   the   emerging  technologies  very  low  C-­‐rates  must  be  used.  Again,  from  a  pack  perspective,  more  cells  in  parallel  are   then  needed   to   fulfil   the  performance   requirements.  Work   is  needed   to  develop  new  materials  and  also  electrode  couples  that  offer  a  significant   improvement  in  energy  and  power  over  today’s  technologies.      The  main  question  to  be  answered  by  this  study  is  whether  there  are  any  potential  post-­‐Li  technologies  to  replace  the  Li-­‐ion  technology  in  electric  vehicle  applications  by  2025.      The   main   research   and   development   needed   is   related   to   the   next   generation   Li-­‐ion  batteries   operating   at   high   voltage   levels   (5V).  Moreover,   cells   having   anodes   of   Si   or  metallic   lithium  will  be   the  most  attractive   solutions   for  electric  vehicles  by  2025  and  therefore   research   should  be   strengthen   for   these   concepts.  Also,   efforts  must   include  the  development  of  novel  electrolyte   formulations  and  additives   to   form  a  stable  solid  electrolyte   interphase   for   improved   abuse   tolerance,   longer   life,   low   temperature  operation,   and   fast   charge   capability.   For   power   demanding   applications,   the   most  attractive  solution  by  2025  will  be  asymmetrical  super  capacitors.    Beyond   2025   the   preferred   technologies   are   Na-­‐ion,   Li-­‐S,   and  Mg,  mainly   due   to   cost  reduction   potentials   of   the   cells   (Na-­‐ion   and   Li-­‐S)   and   two-­‐electron   redox   reactions  (Mg).  Research  efforts  should  be  increased  to  find  the  most  optimal  solutions  for  scaling  up  cells  for  vehicle  applications.    Pack-­‐level  innovations  should  focus  on  technology  to  reduce  the  weight  and  the  cost  of  thermal   management   systems,   structural   and   safety   components,   and   system  electronics.  Currently,  these  “non-­‐active”  components  of  a  battery  increase  the  volume,  weight,  and  cost  of  the  finished  product.  Approaches  to  reduce  the  sizes  of  these  inactive  components   in   the  cell   and  battery  should  be  pursued.  The  cost   reduction  potential   is  highest   for   the   pack   components;   a   potential   of   ca.   75%   resulting   in   a   total   cost  reduction  of  the  battery  pack  by  about  55%  by  2020.    Overall,  this  study  indicates  that  until  2025,  any  huge  improvements  in  the  performance  of  automotive  batteries  are  highly  unlikely  as  there  are  no  game-­‐changing  technologies  approaching  the  consumer  market  today.    

3  

     1   Introduction   4  1.1   Battery  requirements  –  high  level  picture   5  1.2   Boundary  conditions  of  the  study   7  

2   Emerging  battery  technologies  –  Research  trends   8  2.1   Next  generation  Li-­ion   8  2.2   Solid  state  Li-­metal   10  2.3   Na-­ion   15  2.4   Mg   20  2.5   Li-­S   22  2.6   Li-­oxygen   28  2.7   Organic  concepts   33  2.8   Asymmetrical  super  capacitors   34  

3   Emerging  battery  technologies  –  Vehicle  implications   38  3.2   Voltage   39  3.3   Energy  and  Power  density  from  cell  to  pack/system   40  3.4   Cost  trends   42  3.5   Pro’s  and  con’s   44  3.6   Conclusions  and  proposed  actions   45  

4   References   47    

4  

1 Introduction  In  order  to  obtain  electric  vehicles  with  longer  all-­‐electric  driving  range,  the  search  for  batteries  with   higher   energy   density   is   one   of   the   key   issues.   Hybrid   electric   vehicles  rely  on  batteries  having  high  power  capabilities.  Energy  and  power  are  two  properties,  which   are   not   possible   to   optimally   combine   in   one   single   cell   and   therefore   the   cell  selection  is  a  critical  step  in  the  development  of  electrified  vehicles.   It   is,  however,  not  only   energy   and   power   that   is   important   for   long   lasting   batteries   with   high  performance   over   the   entire   life   of   the   vehicle;   durability,   safety,   and   cost   are   other  factors  of   interest.  Today,  most  vehicle  manufacturers  are  using  Li-­‐ion  batteries,  and  a  lot  of  HEVs  are  also  produced  using  NiMH  batteries.      New  improved  electrochemical  active  materials  will  enhance  battery  performance.  The  cells   will   be   further   optimised   along   with   improved   production   processes;   from   raw  materials   to   complete   cells.   Moreover,   the   understanding   of   ageing   mechanisms   to  prolong  the  life  and/or  use  more  of  the  energy  will  most  likely  be  the  main  issue  for  the  vehicle   and   battery   pack  manufacturers   to   enhance   both   calendar   and   cycle   life.   The  production   of   active   materials,   cells,   and   modules/packs   will   be   further   improved   to  reduce  the  cost,  and  to  increase  the  robustness,  capacity  and  safety.          There  are  some  general  routes  to  improve  the  performance,  life,  and  cost  of  battery  cells  and  packs;  summarised  in  Table  1.        Table  1.  Improvement  routes  of  battery  cells  and  packs.     Cell  level   Pack  level  Energy   High-­‐voltage/high-­‐capacity  

materials  (electrodes  and  electrolytes)  

Low-­‐weight  balance  of  plant  components,  Control  strategies  

Power   Electrode  design  (e.g.  3D  design),  Utilisation  of  high-­‐rate  electrode  materials    

Cell-­‐to-­‐cell  connections,  Control  strategies,  Thermal  management  

Life   Understanding  of  degradation  mechanisms  

Thermal  management,  Control  strategies  

Safety   Electrolyte  (salts,  solvents  and  additives),  Separators,  Electrode  coatings  

Thermal  management,  Control  strategies,  Housing,  Electronics,  Vehicle  integration  

Cost   Standardised  cell  formats,  Use  of  low-­‐cost  raw  materials  and  production  processes.  

Modularisation,  Standardised  electrical  components,  Selection  of  optimal  cell  for  specific  vehicles  

   Emerging  battery  technologies  with  new  functional  materials  and/or  concepts  can  also  be   the   route   for   enhanced   battery   performance,   especially   as   the   (theoretical)   energy  density  of  many  emerging  technologies  is  very  attractive.  To  be  an  attractive  technology  for   electric   vehicles   these   emerging   technologies   will   have   to   show   improved  

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performance  on  the  battery  pack  level  and  equal  or  better  cost  in  combination  with  long  life.    As  an  example,   the  Department  of  Energy  (DoE),  US,  has  set  a  high-­‐level  road  map  for  batteries  for  electric  vehicles  pointing  out  examples  of  next  generation  Li-­‐ion  concepts  and  some  emerging  post-­‐Li  technologies.  Their  focus  is  on  volumetric  improvements:      

-­‐ half   of   today’s   volume   by   utilising   graphite/high-­‐voltage   cathodes   (theoretical  energy  560  Wh/kg  and  1700  Wh/L)  

-­‐ a  third  of  today’s  volume  by  Si-­‐based  anodes/high-­‐voltage  cathodes  (theoretical  energy  880  Wh/kg  and  3700  Wh/L)  

-­‐ a  tenth  of  today’s  volume  by  Li-­‐metal  anodes/high-­‐voltage  cathodes  (theoretical  energy  990  Wh/kg  and  3000  Wh/L)  and  by  Li-­‐S  or  Li-­‐O  (theoretical  energy  3000  Wh/kg  and  >3000  Wh/L)    

 Clearly   there   are   limitations  with   this   simple   view;   the   theoretical   values   are   for   cells  and   not   the   corresponding   battery   packs,   and  moreover,   especially   important   from   a  vehicle   perspective,   the   power   capabilities   are   not   included.   Nevertheless,   the   figures  provide  some  concrete  basis  for  how  (large)  improvements  can  be  made.    The   aim   of   the   present   study   is   to   summarise   the   status   of   emerging   battery  technologies  and  their  route  towards  2025  for  vehicle  implementation.  Based  on  a  few  basic  important  parameters  for  electric  vehicles  –  energy,  power,  and  cost  –  the  trends  of  the  emerging  technologies  are  reviewed  and  summarised.  Advantages  and  challenges  for  the  different  technologies  are  compiled  and  recommendations  and  proposed  actions  are   given.  As   there   are  mainly   research   and   laboratory-­‐scale   cells   available   today,   the  review  is  directed  towards  cell  materials,  while  merely  the  foreseeable  implications  for  battery  packs  are  given.  Indeed,  an  attractive  cell  performance  might  be  less  attractive  when  looking  at  the  final  vehicle  installation.    The   main   question   to   be   answered:   Are   there   any   potential   post-­‐Li   technologies   to  replace,  or  complement,  the  Li-­‐ion  technology  in  electric  vehicle  applications  by  2025?      

1.1 Battery  requirements  –  high  level  picture  Vehicle  targets  in  terms  of  all-­‐electric  driving  range  and  fuel  saving  potentials  exist  for  almost  all  regions  and  vehicle  manufacturers.  The  targets  are  often  of  short-­‐term  as  well  as   long-­‐term   character.   In   this   study   the   2025   perspective   is   of   interest,   but   includes  technologies   beyond   2025.   Targets   and   goals   set   by   authorities,   industry,   and  universities  on  vehicle  level,  battery  pack  level,  and  cell  level  are  reviewed  indirectly.  In  the  present  study  the  following  performance  parameters  are  used  for  the  comparison  of  the   different   emerging   battery   technologies:   energy   and   power,   voltage   levels   and  profiles.    

1.1.1 Energy  and  Power  Energy   and   power   densities   are   the   commonly   used   characteristics   employed   for   the  comparison.   Figure   1   summarises   some   publicly   available   battery   pack   targets   from  Japan  (METI),  the  US,  and  EU.    

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 Figure  1.  Energy  and  power  targets  for  a  battery  pack  for  various  needs  from  Japan,  the  US,  and  EU.      Furthermore,   depending   on   type   of   electric   vehicle,   the   power   to   energy   ratio   varies  widely.  Figure  2  indicates  the  ratios  for  different  types  of  electric  passenger  cars  [1].  As  can   be   seen   the   cells   suitable   for   these   applications   are   either   power   or   energy  optimised.  For  comparison,  three  buses  are  added;  BYD  EV  with  a  320  km  range,  Volvo  PHEV  with  a  10  km  all-­‐electric  range,  and  Volvo  full  hybrid.  The  trend  for  the  buses  is  in  line  with  the  trend  for  passenger  cars,  but  most  likely  it  is  not  the  same  cells  that  are  the  most  suitable  for  both  the  passenger  cars  and  the  buses.  

 

 Figure   2.   Power-­‐to-­‐energy   ratios   for   various   types   of   electric   passenger   cars   [1].   For  comparison  three  buses  have  been  added  as  references  (stars).  

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   The   power   capability   of   the   cells   is   one   of   the  most   important   factors   during   the   cell  selection   process.   Energy   optimised   cells   are   associated   with   low   C-­‐rates   and   power  optimised   cells   are   not   suitable   for   energy   demanding   usage.   The   electrode   and   cell  design  highly  affect  the  power  capabilities.  To  understand  the  full  potential  of  emerging  battery   technologies   in   terms   of   power   capabilities   is   difficult   since   most   of   the  technologies   are   still   at   the   research   stage,   i.e.   no   automotive   optimised   cells   are  available.   Indications   are,   however,   available   as   to   whether   or   not   the   emerging  technologies  will  be  suitable  for  use  at  high  C-­‐rates.    

1.1.2 Voltage  levels  and  profiles  For  full  HEVs  and  towards  further  degrees  of  electrification  the  cell  voltage  level  is  not  critical   for  the  total  battery  pack  voltage,  but  will  affect  the  number  of  cells  needed.  In  case  of   lower  voltage   levels,   i.e.  14/24/48  V,  however,   the  cell  selection  can  be  critical  for  the  overall  battery  pack  performance.    The  battery  voltage  will  put  constraints  on  other  parts  of  the  electrical  drive  system  in  electric   vehicles.   Voltage   hysteresis,   i.e.   the   difference   between   the   voltage   profiles  during  charge  and  discharge  of  the  battery,  will  most  likely  affect  the  power  electronics.  Moreover,   voltage  hysteresis   can  be  a   source  of   severe   losses,   especially  during  brake  energy  recuperation.          

1.2 Boundary  conditions  of  the  study  To   cover   all   ongoing   activities  within   the   field   of   rechargeable   batteries   is   impossible  within  the  limited  scope  of  this  project.  Therefore,  some  boundaries  have  been  set.  Only  electrical   rechargeable   battery   technologies   of   interest   in   electrified   vehicles   (from  micro-­‐HEV  to  full  EV)  and  concepts  being  discussed  in  the  vehicle  context  are  described  and   discussed.   Cells   and   cell   materials   are   the   main   objectives.   From   cell   data  implications   on   battery   pack   are   obtained   and   trends   summarised.   The   possible  improvements   of   the   Li-­‐ion   battery   technology   are   also   treated,   but   not   at   the   same  detail   level   as   for   the   emerging   battery   technologies.   For   most   cases,   no   commercial  automotive  cells  are  available  at  the  market  and  therefore  the  battery  pack  performance  will   only   be   indicative.   Moreover,   the   cost   of   the   emerging   battery   technologies   are  rough   estimates   since   the   production   of   various   materials   will   be   the   dominant   cost  driver  for  new  technologies  for  high-­‐volume  production  of  Li-­‐ion  cells,  the  material  cost  can  be  in  the  range  of  50-­‐60%  [1].  Furthermore,  the  life  cycle  and  recycling  perspectives  are  not  included.    

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2 Emerging  battery  technologies  –  Research  trends  In  the  following  sections  the  emerging  battery  technologies  are  described  and  evaluated  for   2025.   The   implication   of   the   basics   of   the   different   technologies   for   vehicle  installation  is  treated  in  Chapter  3.  While  many  of  the  emerging  technologies  are  studied  extensively  at  an  academic  level,  only  limited  vehicle  relevant  data  is  available,  which  is  the  focus  of  this  report.  First,  however,  the  improvement  routes  for  the  next  generation  Li-­‐ion  technology  is  summarised  to  provide  an  appropriate  perspective/base-­‐line.    

2.1 Next  generation  Li-­ion  The  Li-­‐ion  battery  technology  will  be  further  improved  by  advancing  the  performance  of  materials,   designs,   and   processes   aiming   at   the   performance   and   the   cost   of   Li-­‐ion  batteries.   Specific   areas   of   improvements   include   high   voltage   cathodes,   high-­‐energy  anodes   (e.g.   anodes   based   on   Si   or   Sn),   high   voltage   and   non-­‐flammable   electrolytes,  novel  processing  technologies,  high  energy  and  low  cost  electrode  designs,  etc.    

2.1.1 Research  trends  Cathodes:    The   capacity   of   an   active   electrode   material   can   be   increased   by:   i)   increasing   the  average  electrode  potential,  ii)  increasing  the  number  of  electrons  involved  in  the  redox  reactions,   and   iii)   decreasing   the  molecular  weight  per  mole   electrons   exchanged.   For  the   next   generation   Li-­‐ion   batteries  mainly   the   first   route   is   in   focus,   even   if   the   two  latter  are  being  investigated.      The  research  and  development  on  advanced  cathodes  is  primarily  focused  on  the  Li-­‐Mn  rich   oxide   materials   of   general   formula   xLi2MnO3⋅(1-­‐x)LiMO2   (M=Ni,   Mn,   Co),   the   5V  spinel   materials   (e.g.   LiMn1.5Ni0.5O4),   and   Ni-­‐rich   NMC   materials   charged   to   higher  voltages.  The  Li-­‐Mn  rich  materials  have  the  potential  to  give  cells  of  rather  high  energy  density,   about  300  Wh/kg   [2].   To   charge   cells  utilising   the   ‘traditional’  NMC   to  higher  voltage  levels,  for  example  to  4.6  V  instead  of  4.2  V,  would  improve  the  energy  density  by  about  20%.  The  durability  of  such  a  voltage  increase  has,  however,  to  be  secured.  The  electrolyte   stability   and   the   structural   disordering   occurring   during   cycling   of   the  cathode  material  are  issues  to  be  understood.  The  use  of  high-­‐voltage  spinel  materials  is  mainly   limited   by   the   instability   of   the   electrolyte   at   these   voltage   levels.   Surface  coatings,   electrolyte  purity,   additives   to   create   a  more   stable   SEI,   and   additive/binder  free  alternatives  are  routes  forward.  Other  routes  are  doping  of  the  NMC  materials  and  to   increase   the   stability   of   inactive   components   (like   current   collector,   binder,   and  conductive  additive)  at  high  voltages.      The  issues  of  Li-­‐Mn  rich  materials  are  primarily  voltage  fade,  high  impedance  especially  at   low   state  of   charge,  metal   dissolution,   and   low  electrode  density.  By   increasing   the  voltage   level   both   increased   and   improved   capacity   will   be   achieved.   Approaches   to  enabling  higher  voltage  operation   include  varying  the  material  composition  within  the  particles   (for   example   the   outer   material   being   more   stable   against   the   electrolyte),  coatings,  metal  substitutions,  and  electrolyte  additives  that  form  a  protective  coating  on  the  cathode  particles.      

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Other  examples  of  materials  strategies  being  pursued  include  high  voltage  phosphates,  such   as   Li  manganese   phosphate   (LiMnPO4)   and   Li   cobalt   phosphates   (LiCoPO4).   The  LiMnPO4  material  has  a  potential  of  about  0.5  V  higher  vs.  Li/Li+  than  LFP,  which  results  in  a  specific  energy  density  increase  of  about  15  %.  An  even  higher  potential  is  achieved  by   LiCoPO4:   4.8   V   vs.   Li/Li+,   resulting   in   about   40%   higher   energy   density.   The   rate  capabilities   of   these   two  materials   are   poor,   however,   and   LiMnPO4   is   unstable  when  charged  (due  to  the  Jahn-­‐Teller  effect  of  the  MnIII  dominance  in  the  structure).  Moreover,  the   ionic   and   electronic   conductivities,   and   thereby   the   power   performance,   of   both  LiMnPO4   and   LiCoPO4   are   even   considerably   lower   than   those   of   LFP,   making   the  composite  electrodes  much  less  energy  dense.    Cathode  materials  based  on  silicate  chemistry  are  also  promising:  Li2MSiO4,  M=Fe,  Mn,  Co  or  a  mixture  thereof.  The  Li2MSiO4  materials  are  of  interest  due  to  their  (in  theory)  ability   to  electrochemically  extract   two   lithium   ions,   i.e.   double   the   capacity.  Li2FeSiO4  has  a   theoretical   capacity  of   about  330  mAh/g.  These  materials   generally   exhibit  high  temperature  stability  due  to  the  strongly  bound  oxygen  atoms  in  the  SiO4  polyanion,  but  the  overall  performance  is  highly  dependent  on  the  production  process.  It  is  possible  to  extract  more  lithium  from  the  structure  and  thereby  increase  the  capacity  by  means  of  substitution  by  other   transition  metals.  The  most  obvious  choice   is  Mn  by  virtue  of   its  MnII   and   MnIV   oxidation   states.   A   substitution   of   about   20%   results   in   theoretical  capacities   exceeding  200  mAh/g.  However,   to  date   researchers  have  not   succeeded   in  reversibly  extracting  the  second  Li  from  these  materials  and  high  voltage  operation  will  also  be  a  challenge  due  to  the  lack  of  stable  electrolytes.  The  distinct  changes  in  voltage  during  cell  operation  would  be  highly  favourable   in  the  development  of  robust  control  strategies.    Another  group  of  potential   cathode  materials  are   the   fluorophosphates   (LiMPO4F)  and  the   fluorosulphates   (LiMSO4F),   both   often  with   potentials   higher   than   4.5   V   vs.   Li/Li+.  Transition   metals   of   3d   character   are   favoured:  M=V,   Co,   Fe,   Ti,   Mn,   etc.   Even   if   the  molecular   weight   of   the   framework   units   are   higher   than   for   example   NMC,   thus  resulting   in   a   reduced   energy   density,   the   polyanionic   materials   exhibit   very   stable  framework  structures  and  a  wide  range  of  substitution  possibilities.  The  high  electrode  potentials   and   fast   lithium   diffusion,   especially   in   the   case   of   M=V,   are   the   main  advantages,  while  a  general  shortcoming  is  low  electric  conductivities.  An  example  of  an  electrochemically   active   material   that   can   be   used   either   as   positive   or   negative  electrode   is  LiVPO4F.  This  material  exhibits   two  plateaus:  at  1.8  V  and  4.2  V  vs.  Li/Li+,  resulting   in  a   cell   voltage  of  2.4  V.  The   reversible   capacity  at  both  plateaus  are   rather  similar  and  in  the  range  of  150  mAh/g.    Anodes:    The   main   “next   generation”   anode   technologies   to   be   pursued   will   be   alloy   based,  predominantly   silicon   and   tin   based   anodes.   Silicon   based   alloys   are   one   of   the  most  interesting  anodes  concepts  in  terms  of  high  capacity.  The  challenge  is  the  large  volume  expansion   during   the   alloying   reactions   with   lithium.   Research   to   improve   Si-­‐based  anodes   includes:   Cu   foam   current   collectors   to   enable   better   utilization   of   Si   nano-­‐particles,   Si   nano-­‐wires   directly   deposited   on   current   collectors,   a   variety   of   nano-­‐structured   and   nano-­‐porous   Si   materials,   and   a   new   group   of   electrically   conducting  binders  for  use  in  Si  anodes.  All  these  routes  have  the  potential  to  achieve  materials  with  more   than  1000  mAh/g,  based  on  half-­‐cell   experiments.  The  challenge   is   to   tackle   the  

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SEI  stability  and  higher  loadings  that  will  make  the  electrode  structures  more  relevant  to  commercial  batteries.  Other  alloy-­‐based  anodes  are  also  under  the  development.  One  example   is   a   Si/graphene   composite  material   developed  by  Argonne  National   Lab   [3].  Independent  tests  have  shown  from  full-­‐cell  measurements  (with  advanced  cathode  and  electrolyte  materials)  525  Wh/kg  and  specific  anode  capacity  of  1250  mAh/g  [4].      On  a  more  exploratory  front,  some  research  into  conversion  reaction  materials  (e.g.  CoO,  Fe2O3,  and  CuF)   is  performed.  These  materials  provide  high  capacity   (often  more   than  600  mAh/g)   [5].  However,   the   issues  with   these  materials   include:  poor  kinetics,  poor  capacity   retention   on   cycling   (often   due   to   metal   agglomeration),   large   irreversible  capacity  loss,  and  large  voltage  hysteresis.      Electrolytes:    Current  electrolytes,  typically  1M  LiPF6  in  1:1  EC/DMC,  provide  good  performance  and  stability   within   limited   voltage   and   temperature   ranges.   However,   the   solvents   are  highly  flammable  and  typically  have  a  high  vapour  pressure,  which  causes  them  to  gas  at  elevated  temperatures,  building  up  pressure  within  cells  over  time.  Also,  the  LiPF6  salt  is  known  to  react  almost  instantly  with  water,  producing  HF,  which  in  turn  attacks  nearly  all   elements  of   the   cell.  This   reaction,   along  with   the   instability  of  LiPF6  above  ~80°C,  leads  in  part  to  the  challenges  in  Li-­‐ion  cells’  high  temperature  capability.      Work   on   new   electrolytes   and   additives   is   focused   on   one   or   more   of   the   possible  improvement  areas  of  high  voltage  stability,  high  temperature  stability,  low  temperature  operation,  abuse  tolerance,  lower  cost,  and  possibly  longer  life  through  SEI  stabilisation.  Research   areas   include:   flame   retardant   liquid   electrolytes,   single   ion   conductor  electrolytes,  new  salts  providing  better  high  temperature  stability  (for  example  LiTFSI),  and   electrolytes   that   enable  much   lower   temperature   operation.   However,   one   of   the  main  challenges  is  to  find  electrolytes  with  improved  high  voltage  stability;  an  issue  for  example  handled  by  use  of  additives  incl.  ionic  liquids.      Separators:    Current  focus  is  on  developing  separators  that  provide  enhanced  abuse  tolerance,  better  high   voltage   stability,   and   improved   low   temperature   operation.   Some   of   the  technologies   being   developed   include   a   ceramic   impregnated   separator   that   shows  much  improved  low  temperature  performance  and  greatly  increased  high  temperature  melt   integrity.   The   latter   may   be   important   to   the   avoidance   of   shorts   during   high  temperature   excursions   that   can   occur  when   traditional   separators   shrink.  Another   is  developing   a   separator   and   process   to   permit   direct   deposition   onto   anode   and/or  cathode  sheets.      

2.2 Solid  state  Li-­metal  The   development   of   long-­‐life-­‐cycling   lithium   batteries   was   initially   based   on  metallic  lithium  anodes  together  with  suitable  solid  electrolytes.  Metallic  lithium  has  a  very  high  electronegativity  while  possessing  the  lowest  density  amongst  all  metals,   leading  to  its  high   specific   capacity   (3861   mAh/g)   and   has   thus   been   considered   to   be   the   best  candidate   for  rechargeable  Li-­‐battery  anodes.  The  key   for   this  battery  technology   is   to  find  an  electrolyte  stable  towards  the  metallic   lithium  anode  and  to   inhibit  Li-­‐dendrite  formation  during  cycling.  

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 Compared   with   liquid-­‐electrolyte   lithium   batteries,   the   bottleneck   of   solid-­‐state  batteries   is   their   poor   performance   under   high   power,   resulting   from   the   low   ionic  conductivity  of   the   solid  electrolyte,   the  electrode/electrolyte   interfacial   compatibility,  and   limited   kinetics   of   the   electrodes.   Reasonable   rate   capabilities   can,   however,   be  achieved   by   utilising   a   thinner   electrolyte,   good   interfaces,   and   fast   kinetics   of   the  electrodes.      There   are   two   general   classes   of   materials   used   as   electrolytes   in   all-­‐solid   state  batteries:   inorganic   ceramics/glasses   and   organic   polymers.   The   most   obvious  difference   between   these   materials   is   their   mechanical   properties.   The   high   elastic  modulus   of   ceramics   and   glasses  makes   them  more   suitable   for   rigid   battery   designs,  and   the   low   elastic  modulus   of   polymers   is   useful   for   flexible   batteries.   Polymers   are  also  generally  easier  to  process  than  ceramics,  especially  at  lower  temperatures,  which  reduces   the   production   costs.   On   the   other   hand,   ceramics   are  more   suitable   for   high  temperatures,   high   voltages,   and   aggressive   environments.   The   anode   and   the  electrolyte   are   the   main   issues   for   this   type   of   battery   technology.   Therefore,   the  research  trends  for  metallic  lithium  anodes  and  solid  electrolytes  will  be  given.    The  BlueCar   by  Bolloré   is   equipped  with   Li-­‐metal   polymer   batteries.  More   than   2000  cars   are   running   in   the   centre   of   Paris   in   the   car-­‐sharing   programme   Autolib’.   The  battery  packs  have  an  energy  density  of  100  Wh/kg  or  100  Wh/L  [6]  and  the  cars  have  a  driving  range  of  250  km  on  one  charge  with  a  C-­‐rate  of  C/4.    

2.2.1 Research  trends  Anode:  The  main  issue  using  metallic  lithium  as  anode  is  the  dendrite  formation  on  the  lithium  anode   during   repeated   charge/discharge   cycles,   which   can   cause   internal   short  circuiting  and,  thus,  a  severe  safety  concern.  Low  Coulombic  efficiency  is  another  issue  facing  the  lithium  electrode.  It  has  been  proposed  that  the  continuous  growth  of  the  SEI  on   the   lithium  electrode   surface   under   uneven   current   distributions   and   formation   of  irreversible  “dead  lithium”  are  responsible  for  the  dendrite  formation  [7].  Stabilising  the  surface   of   the   metallic   lithium   anode   has   been   proposed   both   by   mechanical   and  chemical   means.   To   find   a   stable   Li-­‐anode   is   not   only   of   interest   for   solid-­‐state   Li  batteries,  but  also  for  the  Li-­‐S  and  Li-­‐O2  battery  technologies  (see  2.5  and  2.6).    The  chemical  stability  of  lithium  on  a  metal  substrate  is  also  a  major  issue  for  the  self-­‐discharge   behaviour.   The   chemical   stability   of   deposited   lithium   has   been   studied   by  understanding  the  corrosion  process  [8]  and  mechanisms  have  been  proposed  [9].    To  protect  the  metallic  Li  surface  is  another  route  forward.  One  example  is  to  introduce  an  interfacial  layer  of  hollow  carbon  nano-­‐spheres.  Stable  cycling  results  using  a  current  density  of  1  mA/cm2  has  been  achieved  with  a  capacity  of  1  mAh/cm2  and  a  Coulombic  efficiency  of  ca.  99%  for  more  than  150  cycles  [10].  Electrolyte  additives  may  also  play  an  important  role  in  the  stability.      The  effect  of  mechanical   surface  modification  on   the  performance  of   lithium  has  been  investigated   by   utilising   micro-­‐needle   surface   treatment   techniques   [11].   The  mechanically  modified   surface  was   shown   to   improve   the   rate   capability   by  20%  at   a  

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rate  of  7C.  Moreover,   the  cycling  stability  was   increased  by  200%  showing  85%  of  the  initial  discharge  capacity  after  150  cycles,  compared  to  untreated  bare  Li  metal  showing  85%   of   the   initial   discharge   capacity   after   only   70   cycles.   This   technique   has   been  suggested   to   suppress   Li   formation   of   high   surface   area   Li   during   the   Li   deposition  process,   as   preferred   sites   for   controlled   Li   plating   are   generated.   Stabilised   Li-­‐metal  powder  is  another  example,  which  has  been  evaluated  mainly  in  Li-­‐ion  cells  [12],  where  results  indicate  that  the  powder  could  be  used  as  an  independent  source  of  lithium.    Electrolyte:  There  are  several  types  of  possible  solid-­‐state  electrolytes  for  all-­‐solid  state  Li-­‐batteries.  In  the  following  the  ceramic  and  polymeric  electrolytes  are  reviewed.  A  common  issue  for   these  electrolytes  are   the   low   ion  conductivity  at   room   temperature  and   therefore  the  need  of  elevated  operational  temperatures.    Ceramic  Many   ceramic   electrolytes   have   a   voltage   window   beyond   5   V,   and   thus   do   not  decompose   under   anodic   current,   such   as   Li10GeP2S12,   [13]   Li3PS4,   [14]   Li4SnS4,   [15]  Li7La3Zr2O12,   [16]   and   amorphous   lithium   phosphorus   oxynitride   (LiPON)   [17].  Furthermore,  with  a  solid  electrolyte,   the  concern  of   transition  metal  dissolution   from  the   electrodes   into   the   electrolyte   is  minimal.   Compared  with   liquid   carbonate-­‐based  electrolytes,  most   solid  electrolytes  are   intrinsically  non-­‐flammable.  Moreover,   lithium  metal   is   compatible   with   many   solid   electrolytes   and   is   less   likely   to   form   dendrites  during  cycling  because  of  the  mechanical  robustness  of  the  solid  electrolyte  [18].      One   widely   studied   type   of   solid   electrolytes   is   the   NASICON   structured   compounds:  AxMM’(XO4)3   already   identified   in   the   late   60’s   [19]   (A=   alkaline  metal,   Na   originally,  M=transition  metal(s)).  The  structure  is  a  three-­‐dimensional  network  of  interconnected  conduction  channels  and  two  types  of  interstitial  positions  where  conducting  cations  are  distributed.  The  number  of  alkali  cations  (x)  per  structural  formula  AxMM’(XO4)3  can  be  adjusted  depending  on   the  oxidation  states  of   the   transition  metals  and   the  element  X  and  the  large  interstitial  space  can  accommodate  up  to  5  alkali  cations  per  formula  unit  [20,21].  The  structural  and  electrical  properties  of  NASICON  type  compounds  vary  with  the  composition.  By  substitution  of  trivalent  cations  (e.g.  Al,  Cr,  Ga,  Fe,  Sc,  La)  for  Ti4+  in  the  octahedral  sites,  the  Li  ion  conductivity  in  can  be  improved  [22,23].    Adding  B2O3  to  LiTi2(PO4)3   ceramics   has   been   investigated   and   the   ionic   conductivity   at   room  temperature  was   significantly   enhanced   [24].  The  boron  oxide   acts   also  as   a   sintering  aid  to  reduce  the  grain  size,  while  enhancing  the  contact  within  the  solid  electrolyte.  At  a  larger  B2O3  content  the  Li-­‐ion  diffusion  is  limited  at  the  grain  boundaries.  NASICON-­‐type  compounds   are   stable   with   Li   or   Na  metal   electrodes   only  when   reducible   transition  elements  are  absent.  An  example  of  NASICON  compounds  have,  however,  been  reported  without   any   reducing   element   [25].   However,   this   compound   has   a   very   low   Li-­‐ion  conductivity   (around   10−8   S/cm)   at   room   temperature   and   is   only   stable   above   50°C.  The  substitution  of  Ca  for  Zr  in  LiZr2(PO4)3  introduces  additional  Li+  ions  and  stabilizes  the   structure   at   room   temperature.  The   room-­‐temperature  bulk  Li-­‐ion   conductivity  of  Li1.2Zr1.9Ca0.1(PO4)3  approaches  1.2*10-­‐4  S/cm  [25].  A  recent  very  complete  overview  of  NASICON  structures  can  be  found  in  [26].    Another   type   of   ceramic   electrolytes   is   the   LISICON   compounds   (Li2+2XZn1-­‐XGeO4)  [27,28].   The   LISICON   framework   has   a   relatively   low   conductivity   (about   10-­‐6   S/m   at  

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room   temperature).   Furthermore,   LISICON   structures   can   be   highly   reactive   with  lithium   metal   and   atmospheric   CO2   and   the   conductivity   decreases   with   time.   The  structure  can,  however,  be  improved  by  substituting  some  oxide  ions  by  larger  and  more  polarisable   sulphide   ions   -­‐   the   thio-­‐LISICON   family   [29,30].   A   recently   reported   new  lithium   superionic   conductor,   Li10GeP2S12   in   the   thio-­‐LISICON   family   has   a   three-­‐dimensional   framework   structure   and   exhibits   an   extremely   high   lithium   ionic  conductivity  of  1.2*10-­‐2  S/cm  at  room  temperature  [31].  This  represents  the  highest  Li-­‐ion  conductivity  achieved   in  a  solid  electrolyte,  exceeding  even   those  of   liquid  organic  electrolytes.  By  ab  initio  calculations  and  MD  simulations  [32]  the  ion  conductivity  has  been   explained   and   the   compound   is   in   fact   a   metastable   phase   not   stable   towards  reduction  by  lithium.  The  calculated  band  gap  is  3.6  eV  and  the  reported  electrochemical  stability  window  (higher  than  5  V)  is  likely  the  result  of  a  passivation  phenomenon.      Another   family   of   Li-­‐ion   conducting   oxides   with   garnet-­‐related   structure   (general  formula  Li5La3M2O12  (M=Ta,  Nb))  is  currently  being  investigated  [e.g.  33].  These  oxides  exhibit   pure   lithium   ion   conductivity   and   a   wide   electrochemical   stability   window.   A  lithium   ion   conductivity   at   room   temperature   of   about   4*10-­‐5   S/cm   was   obtained   in  barium  doped  samples,  Li6La2BaTa2O12  [34,35].    Laboratory-­‐scale  cells  made  of  LiNi0.5Mn1.5O4//LIPON//Li  have  been  cycled  between  5.1  V   and   3.5   V   vs.   Li+/Li   at   rates   of   5C,   and   a   high   Coulombic   efficiency   of   99.98%  was  achieved  [36].  This  indicates  that  the  decomposition  of  the  solid  electrolyte  is  minimal.  The  charge  loss  over  1000  cycles  for  the  tested  cells  were  about  125  times  smaller  than  in  corresponding  liquid-­‐electrolyte  cells.    Polymeric  Polymer   electrolytes   offer   several   advantages   over   ceramics,   including   good  processability  and  flexibility,  and  exhibit  several  attractive  properties   like  dimensional  stability,  safety  and,  in  most  cases,  the  ability  to  prevent  lithium  dendrite  formation  [37-­‐39].   Furthermore,   their   ability   to   deform   elastically   and   plastically   are   suitable  properties  for  all-­‐solid  batteries,  allowing  efficient  interfaces  towards  the  electrodes  and  for  volume  changes  taking  place  during  cycling.  For  reviews  of  the  early  developments  in   this   field   please   see   [40-­‐43].   The   ion   mobility   is   associated   with   local   structural  relaxations  of   the  polymer.   In   solid  polymer  electrolytes,   lithium  salts   are   solvated  by  the  polymer  chains,  while  in  others  a  solvent  is  added  to  form  a  polymer  gel,  but  those  require   a  mechanical   support   and  will   not   be   discussed   here.   A   recent   review   can   be  found  in  [44].      The   most   commonly   used   polymer   for   lithium-­‐ion   conducting   solid   electrolytes   is  poly(ethylene  oxide)  (PEO),  in  which  Li-­‐salts  are  effectively  dissolved.  The  conductivity  of  PEO  with  various  lithium  salts  is  of  the  order  of  10-­‐5  -­‐  10-­‐6  S/cm  at  room  temperature  with  the  highest  values  observed  for  LiTFSI  as  the  salt  [38],  but  further  improvement  is  needed  for  room  temperature  battery  applications.  The  activation  energy  for  lithium-­‐ion  conduction  in  PEO  decreases  with  increasing  temperature.  The  ionic  conductivity  of  PEO  is  essentially  due  to   transport   in   the  amorphous  regions,  so   the  conductivity  generally  decreases  with   increasing  degree  of  crystallization,  but  cation  conductivity  can  be  also  observed  in  crystalline  phases  [45].    

14  

PEO  is  stable  towards  metallic   lithium  and  batteries  employing  PEO  based  electrolytes  have  been  demonstrated  to  have  capacities  similar  to  batteries  with  the  commonly  used  liquid  electrolytes.  One  general   issue  with  polymer  electrolytes   is   that  the  anions  have  higher  transference  numbers  than  the  cations  due  to  ‘trapping’  of  cations  at  the  polymer  chain   [e.g.   46].   One   strategy   to   enhance   the   transference   number   of   the   cations   is   to  increase  the  volume  and  mass  of  the  anion;  why  very  bulky  and  heavy  anions  have  been  designed,  but  without  definitive  success.    Adding  ceramic  particles  –  fillers,  such  as  LiAlO2,  alumina,  titania,  NASICON,  or  silica,  in  composite  polymer  electrolytes  (CPE)   leads   to  an   increased   ion  conductivity,  probably  related  to  a  decrease  of  crystallization  [47-­‐50].  The  change  of  the  ionic  conductivity  with  the   filler   content   is   non-­‐linear   with   a   maximum   in   the   range   5–15   wt%   of   filler,  depending  on  the  polymer  matrix,  the  lithium  salt  used,  and  the  nature  of  the  filler.  The  effect  of  process  conditions,  type  of  fillers  and  size,  has  also  been  investigated  [51].  The  addition  of  ceramic  particles  can  also  improve  the  mechanical  properties  of  the  polymer,  which   is   important   in   designing   a   polymer   electrolyte,   because   most   changes   that  increase  conductivity  are  detrimental  to  the  mechanical  performance.  Another  route  has  been   to   investigate   polymers   within   porous   ceramic   alumina,   the   reverse   of   the   CPE  [52,53].      In   addition   to   modifying   PEO   in   composites,   alternative   solid   polymer   electrolyte  materials  have  been  developed  having  high  mechanical  strength,  such  as  acrylate-­‐based  electrolytes  [38].  Examples  of   these  electrolytes  are  poly(ethylene  oxide)-­‐methyl  ether  methacrylate   (PEOMA)   [54],   polystyrene-­‐block-­‐poly(ethylene   glycol)   methyl   ethyl  methacrylate   (PEGMA)   [55].   Block   copolymers   have   also   been   described   as   possible  solid   polymer   electrolytes,   where   for   example   polystyrene   blocks   improve   the  mechanical   behaviour   of   the   membranes   keeping   acceptable   lithium   ion   conductivity  [56].  Different   conduction  mechanisms   can   dominate   in   different   temperature   ranges,  and  by  mixing  polymers  the  operating  temperature  of  the  electrolyte  can  be  expanded.  Li-­‐ion   conductivity   can   also   be   enhanced   by   forming   lamellar   structured   block  copolymers,   where   blocks   can   be   ionophilic   and   the   ionic   conductivity   increased   at  higher  temperature  [57].    A  different  approach  is  to  fix  the  anion  on  the  macromolecular  chain.  In  these  polymers  the  transference  number  of  the  lithium  cation  is  expected  to  be  unity  so  that  polarisation  losses   due   to   anion   migration   can   be   avoided.   One   example   is   short   side-­‐chain  perfluorinated  sulphonic  acid  (Hyflon)   ionomer,  using   lithium  hydroxide   in  absence  of  organic   solvent   [58,59].   Another   attractive   electrolyte   is   Li-­‐poly(4-­‐styrenesulfonyl(trifluoromethylsulphonyl)imide)   (PSTFSI),   containing   –SO2-­‐N-­‐-­‐SO2-­‐CF3  anions,   attached   to   a   polystyrene   chain,   which   are   associated   with   a   lithium   counter  cation.  An  ionic  conductivity  of  10-­‐6  S/cm  has  been  measured  at  room  temperatures  for  PEO/PSTFSI  composites  [60].      The  interfacial  instability  between  the  electrode  and  electrolyte  is  a  great  challenge  for  solid-­‐state  batteries   [13,15,61,62],  and  proper  engineering  of   the  electrode/electrolyte  interfaces  is  ultimately  required  for  acceptable  cycling  performance  of  most  solid-­‐state  lithium  batteries  [13,61,63,64].    

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2.3 Na-­ion  Based  on  the  same  basic  principles  as  the  Li-­‐ion  concept  other  types  of  metal-­‐ion  (Me-­‐ion)   concepts   are   possible.   The   Me-­‐ion   concepts   of   relevance   depend   on   the  electrochemical  capacity  and  the  operating  voltages.        Na-­‐ion  is  one  of  the  most  attractive  Me-­‐ion  candidates  and  the  concept  is  comparable  to  the  Li-­‐ion  concept  in  very  many  aspects;  the  voltage  levels  are  in  the  same  range  and  the  energy   density   is   comparable   with   Li-­‐ion   batteries   and   thus   of   interest   for   electric  vehicle   applications.   Sodium   is   three   times   heavier   than   lithium   (23   g/mol   and   6.9  g/mol,  respectively)  and  is  0.3  V  less  electropositive,  so  relatively  high  gravimetric  and  volumetric  capacity  penalties  (ca.  15%)  may  have  to  be  paid  in  moving  from  lithium  to  sodium  batteries.  Yet,   the  0.3  V  difference   is  based  on  the  metals  and  not  on  the   ‘true’  anode  materials  used.  Moreover,  the  availability  of  sodium  in  the  Earth’s  crust   is  more  than   1000   times   higher   than   that   of   lithium,   resulting   in   a   more   solid   sustainability  perspective   and   long   term   cost   competitiveness   for   the   Na-­‐ion   concept.   Another  advantage  of  Na-­‐ion  cells  compared  to  Li-­‐ion  cells  is  the  fact  that  Na  does  not  form  alloys  with  aluminium,  hence  aluminium  can  be  used  as  current  collectors  for  both  electrodes,  resulting  in  a  lower  total  weight  (and  material  cost)  of  the  Na-­‐ion  cell  compared  to  the  Li-­‐ion  cell  (avoid  heavy  and  expensive  copper).  A  7%  weight  reduction  of  the  cell  can  be  expected.  If  the  Na-­‐ion  technology  could  be  achieved,  early  estimates  predict  a  30%  cost  decrease   of   the   cell  materials   (incl.   Cu   to   Al)  with   respect   to   Li-­‐ion   technology  while  ensuring  sustainability  [65].  Such  a  cost  reduction  also  takes  into  account  the  possibility  to  develop  cheaper  sodium-­‐based  electrolytes.    The  principle  of  cell  operation  is  the  same  as  its  Li-­‐ion  cousin:  sodium  ions  are  shuttled  between  the  cathode  and  anode  through  a  non-­‐aqueous  (or  aqueous)  electrolyte.  During  charge,   sodium   ions   are   extracted   from   the   high   voltage   positive   electrode,   with   a  working  potential  around  or  above  3.0  V  vs.  Na/Na+  (see  Figure  3  for  potential  electrode  materials),   and   are   inserted   into   the   low   voltage   negative   electrode,   whose   working  potential  is  ideally  lower  than  1.0  V  vs.  Na/Na+  (Figure  3).    

 

   Figure  3.  Electrode  potential  of  some  suitable  active  electrode  materials  for  Na-­‐ion  cells  [66].    

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2.3.1 Research  trends  Cathode:  Cathode  materials  of  interest  are  primarily  those  having  low  activation  polarisation  for  Na+   transport   and   small   volume   changes   during   insertion   and   extraction.   The   main  groups  of  material  for  the  cathode  are,  as  for  Li-­‐ion  cells,  layered  transition  metal  oxides  and   polyanionic   framework   structures.   The   layered   compounds   are   often   of   NaxMO2  (x≤1)   insertion   character,   where  M   is   a   transition   metal,   e.g.   Mn,   Co,   Fe,   or   Cr,   or   a  mixture  thereof.  The  electrochemical  properties  of   layered  NaMnO2  have  been  showed  that   0.8   Na   can   be   reversibly   cycled   with   good   capacity   retention,   equivalent   to   a  capacity  of  200  mAh/g  [67].  The  voltage  profiles  exhibit  pronounced  stepwise  processes  indicative  of  structural  transitions.    Maybe   the  presently  most   promising   cathode  material,   in   terms  of   both   sustainability  and  electrochemical  performance,  is  Na2/3Mn1/2Fe1/2O2  [68].  In  addition  to  the  low  cost  of  manganese  and  iron,  the  material  is  attributed  with  a  high  specific  capacity  of  about  190  mAh/g  and  a  specific  energy  over  520  Wh/kg  based  on  half-­‐cell  measurements  [68].  This   is   comparable   to   LiFePO4,   which   exhibits   a   practical   cathode   energy   density   of  about  530  Wh/kg.  Moreover,  cell  retains  about  70%  of  its  reversible  capacity  when  the  cycling   rate   is   increased   from   C/20   to   1   C.   The   superior   rate   capability   of  Na2/3Mn1/2Fe1/2O2   compared   to   that   of  many   other   layered   transition  metal   oxides   is  correlated   to   its   smooth   charge/discharge   voltage   profile,   which   suggests   a   lack   of  pronounced   structural   transitions   during   cycling.   At   higher   potentials,   the   Na-­‐based  material  is  not  subject  to  detrimental  oxygen  release,  which  is  common  for  the  layered  LiMO2   materials.   Furthermore,   full-­‐cell   data   using   hard   carbon   anodes   have   shown  capacities  of  100  mAh/g  for  150  cycles  in  the  voltage  range  of  1.5-­‐4  V  at  a  rate  of  0.5  C  [69].  The  capacity  of  such  a  cell  is  limited  by  irreversible  processes  associated  with  the  carbon  negative  electrode  that  emerge  from  the  formation  of  an  SEI.    Layered   oxides   utilising  magnesium   substitution   (e.g.   Na2/3Mn4/5Mg1/5O2)   have   shown  discharge   capacities   of   150-­‐220   mAh/g   between   1.5   and   4.5   V   with   an   excellent  retention   of   capacity   (>96%)   [70,71].   A   large   fraction   of   this   reversible   capacity   is  associated  with  a  well-­‐defined  voltage  plateau  at  4.2  V.    In  the  search  for  cathode  materials  with  stabilities  acceptable  for  practical  Na-­‐ion  cells,  studies   have   been   performed   on   the   effect   of   Li   substitution   on   the   structural   and  electrochemical  properties  of  a  layered  material,  Nax[LiyNizMn1-­‐y-­‐z]O2  (0<  x,y,z<1)  [72].  A  smooth  voltage  profile   over   the   entire   range  of   cycling  was  observed,  which   indicates  sodium  de/intercalation  through  a  solid-­‐solution  process.  Cells  with  capacity  within  the  range   of   115-­‐200  mAh/g  have  been   shown,  when   cycled   between  2.0   and  4.4  V,  with  acceptable  retention  (91%  after  50  cycles)  and  rate  capability  [72,73].    Despite   the   advantages   that   layered   sodium   transition   metal   oxides   offer   for  electrochemical  energy  storage  applications,  their  air  sensitivity  is  a  challenge.  This  is  an  important  issue  in  terms  of  the  reproducibility  of  results  from  research  studies  on  these  materials,  in  addition  to  concerns  of  storage  and  handling  from  a  large-­‐scale  application  point  of  view.  Therefore,  other  types  of  cathode  materials  are  also  being  investigated.      The  polyanionic   framework  structures,  often  containing  PO4  groups,  either  alone  or   in  combination   with   F,   enables   of   low-­‐energy   Na+   migration   pathways,   possibilities   of  tuning   the   operating   voltage   by   modifying   the   local   environments,   and   favourable  

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structures   for   a   flat   voltage   response   offer   some   crucial   advantages.   In   addition,   their  robust   covalent   frameworks   render   them   thermally   stable   and   ensure   impressive  oxidative   stability   at   high   charging   voltages.   NaFePO4,   unlike   its   lithium   analogue  olivine-­‐type  LiFePO4,  normally  crystallises  in  thermodynamically  more  stable  structure  (triphylite   or   maricite)   without   diffusion   pathways   for   Na   ions   [74-­‐76].  Electrochemically   active   olivine-­‐type  NaFePO4  has   been  prepared  by   low-­‐temperature  Li/Na  exchange  from  LiFePO4.  This  electrochemically  active  NaFePO4  has  shown  about  100  mAh/g  at  1C  rate  [77].    A  more  promising  group  of  cathodes  materials  are  the  NASICON  compounds,  having  an  open   3D   frameworks   enabling   fast   conduction   of   Na   ions.   These   compounds   were  initially  explored  as  solid  electrolytes  [78]  and  only  more  recently  as  insertion  materials.  Amongst  the  various  NASICON  compounds,  Na3V2(PO4)3  has  emerged  as  an  interesting  candidate  because  of  its  impressive  energy  density  (400  Wh/kg)  and  thermal  stability  in  the  charged  state   [79].  The  corresponding  voltage  profile  has   two  voltage  plateaus   (at  3.4  V  and  1.6  V)  corresponding  to   the  V3+/V4+  and  V2+/V3+  redox  couples,  respectively.  Only   the   higher   voltage   couple   is,   however,   suitable   for   a   cathode   material   or   the  material  could  be  used  as  both  anode  and  cathode.  The  material  has  a  poor  electronic  conductivity,   and   therefore   nano-­‐structured   materials   embedded   in   a   matrix   of,   for  example   porous   carbon   or   carbon   nano-­‐fibres,   are   needed   to   achieve   the   acceptable  capacity  at  practical  current  rates  [80,81].      Inclusion  of   fluorine  atoms   in   the   covalent  polyanionic   framework  has  been   shown   to  improve   the  voltage  of   the  active  redox  couple.  One  promising  example   is   the   fluorine  containing   NASICON   analogue   Na3V2(PO4)2F3   (NVPF)   [82]   due   to   its   high   average  voltage  of  3.9  V  [83].  Other  examples  include  Na2FePO4F  and  Na1.5VPO4.8F0.7.  The  former  has   a   voltage   of   2.90-­‐3.05   V   and   a   theoretical   capacity   of   120   mAh/g     [84].   Despite  pathways   available   for   fast   Na   ion   mobility,   the   electrochemical   kinetics   is   not   as  favourable   as   in   Na1.5VPO4.8F0.7,   which   also   exhibits   a   layered   structure.   The   latter  compound   has   demonstrated   the   attractive   cycling   performance,   with   95%   and   84%  capacity   retention   after   100   and   500   cycles,   respectively,   at   1C   rate   and   negligible  overpotential  throughout  the  charge/discharge  process  [77].    Anode:  Na-­‐ion  cells  do  not  employ  metallic  sodium  as  anode.  This  is  mainly  due  to  the  formation  of  dendrites  and  the  safety  issues  related  to  the  usage  and  handling  of  metallic  sodium  having   a   melting   point   of   only   98   °C.   Thus,   the   success   of   the   Na-­‐ion   technology   is  strongly   dependent   on   the   development   of   safe   and   efficient   anode   materials.   Hard  carbons   or   metal   oxide   intercalation   compounds   are   of   main   interest,   but   alloys   or  conversion  reaction  materials  may  be  used.  Graphite,  as  used   in  Li-­‐ion  cells,  cannot  be  made   to   electrochemically   incorporate   Na+   ions   into   the   host   structure   [85,86].  Therefore,   disordered   carbon   materials,   with   diverse   morphologies,   microstructures  and  degrees  of  graphitisation,  are  used  for  Na-­‐ion  anodes.   In  hard  carbon  the  Na+   ions  are  inserted  into  nano-­‐pores  between  randomly  stacked  graphene  layers  and  reversible  capacities  up  to  300mAh/g  have  been  achieved  [87].  The  promising  performance  of  hard  carbon  as  a  negative  material  for  Na-­‐ion  cells  has  been  demonstrated  in  full  cells  using  NaMn1/2Ni1/2O2  as  the  positive  electrode  [88]  and  NVPF  [82].  Cycling  Na-­‐ion  cells  of  hard  carbon  anodes  at  a  high  C-­‐rates  and/or  to  low  state  of  charge  levels  may  result  in  safety  issues,   however,   since   the   insertion   potential   is   very   close   to   the   sodium   plating  

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potential.   To   overcome   these   issues   porous   hard   carbon   prepared   based   on   a   silica  templating   approach   has   been   reported   [89].   The   high   porosity   and   advanced  microstructure  enhance  the  high  rate  capability,  thereby  resulting  in  a  capacity  of  about  180mAh/g  at  a  rate  of  C/5.    Furthermore,  insertion  of  sodium  in  expanded  graphite  has  been   reported   [90,91].   The   distance   between   the   graphene   layers   strongly   influences  the  reversible  capacity  and  capacities  up  to  280mAh/g  has  been  shown  with  a  capacity  retention   of   more   than   70%   after   2000   cycles,   which   suggests   that   the   expanded  graphite  anodes  are  attractive  from  a  durability  perspective  [90].  The  insertion  potential  of   these   materials   is   a   sloping   curve   from   1.5   V   to   0   V   with   more   than   80%   of   the  capacity   under   1   V.   The   higher   voltage   compared   to   hard   carbon   thus   increases   the  safety  at  the  expense  of  the  energy  density.    As   for   Li-­‐ion   cells,   anode   materials   based   on   oxides   are   of   interest   for   Na-­‐ion   cells.  Na2Ti3O7   exhibits   a   particularly   low   potential   desirable   for   Na   insertion   [92].   The  insertion  of  two  additional  sodium  atoms  occurs  at  a  reversible  plateau  around  0.3  V  vs.  Na/Na+  and  corresponds  to  about  180  mAh/g.  Very  slow  rates  (about  C/25)  have  to  be  used   to   achieve   this   capacity   and   a   composite   electrode   with   30%   carbon   black   is  necessary;   a   decreased   energy   density   will   result   and   the   carbon   black   is   also  responsible  for  large  irreversible  capacity  losses  of  the  same  order  of  magnitude  as  the  reversible  capacity  observed  on  the  first  cycle  [92].  The  rate  capability  has  recently  been  significantly   improved   by   reduction   of   the   particle   size.   Reversible   capacities   of   110  mAh/g  at  4  C  and  75  mAh/g  at  5  C  have  been  shown  [93,94].  Another  example  of  oxides  as  anode  materials  is  Na0.66Li0.22Ti0.78O2,  showing  a  reversible  capacity  of  ca.  120  mAh/g  at  C/10  with  an  average  voltage  of  0.7  V  vs.  Na/Na+  and  a  rate  capability  of  75  mAh/g  at  1   C   and   75   %   capacity   retention   after   1200   cycles   [95].   The   basically   attractive  performance   is   attributed   to   the   very   small   volume   change   (0.8%)   upon   sodium  insertion.  Overall,  the  performance  of  these  anodes  is,  however,  not  at  present  attractive  for  vehicle  applications  and  further  research  is  needed.    Other  types  of  negative  electrode  materials  are  different  alloys,  mainly  based  on  Sn,  Sb,  or   Sn/C.   The   large   ionic   radius   of   the  Na+   ion   is,   however,   expected   to   result   in   large  volume  changes  upon  formation  of  sodium  alloys.  With  an  average  voltage  of  0.3  V  vs.  Na/Na+  and  a   theoretical  capacity  of  790  mAh/g,  Sn   is  a  promising  alloying  candidate.  Despite   a   volume   expansion   of   420%,   composite   electrodes   of   Sn   powder   with   a  polyacrylate  binder  show  a  reversible  capacity  of  500mAh/g  over  20  cycles,  though  at  a  slow  cycling  rate  [96].  A  larger  reversible  capacity  of  about  600mAh/g  over  160  cycles  at   a   rate   of   C/10  with   an   average   voltage   of   0.8   V   vs.   Na/Na+   has   been   reported   for  microcrystalline   Sb   [97].   The   rate   capability   of   this   material   is   attractive,   showing   a  reversible  capacity  of  500mAh/g  at  4  C  rate.  Moreover,  Na  insertion  into  amorphous  P  is  attributed   with   a   high   capacity   of   1500mAh/g   during   first   cycles   at   C/10,   with   an  average  potential  of  0.6  V  vs.  Na/Na+.  About  1000mAh/g  retains  after  80  cycles,  and  at  higher  rates  (1C)  a  reversible  capacity  of  1000mAh/g  has  been  observed  [98].    Electrolyte:  With  the  respect  to  the  electrolyte,  Na-­‐ion  cells  have  the  same  demands  as  every  other  battery   concept   i.e.   it   must   be   stable   in   the   whole   voltage   range   in   order   to   secure  durability   and   safe  usage.  The  electrolytes  used   in  Na-­‐ion   cells   are  very   similar   to   the  ones   used   in   Li-­‐ion   cells.   Only   non-­‐aqueous   electrolytes   are   considered   due   to   the  voltage   levels  needed  for  vehicle  applications.  All  current  non-­‐aqueous  electrolytes   for  

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Na-­‐ion   cells   are   based   on   carbonate   solvents,   such   as   ethylene   carbonate   (EC)   and  propylene   carbonate   (PC)   because   of   their   very   high   dielectric   constants,   large  electrochemical  stability  windows,  and  low  volatilities.  PC  has  become  the  main  solvent  used  for  Na-­‐ion  cells  and  is  the  base  of  about  60%  of  the  electrolyte  formulations  in  the  Na-­‐ion  battery   literature,   a   notable   difference   compared   to   Li-­‐ion  batteries  where   the  use   of   graphite   anodes   prohibits   the   application   of   PC.   Nonetheless,   pure   PC   based  electrolytes   seem   to   induce   strong   capacity   fading   and   low   Coulombic   efficiencies   for  hard  carbon  negative  electrode  materials  [88].  EC  has  been  introduced  as  co-­‐solvent  and  is   becoming   a   key   player   since   it   promotes   a   more   stable   SEI   also   for   hard   carbon  electrodes,   likely   related   to   the   formation   of   ether   functionalities   upon   reduction  [99,100].    The  electrochemical  operation  window  for   the  Na-­‐ion  technology   is  approximately   the  same  as  for  Li-­‐ion  and  thus  the  range  of  suitable  salt  anions  are  similarly  limited  by  their  oxidation  and  reduction  properties.  Sodium  salts  (based  on  the  anions  PF6−,  TFSI−,  FSI−)  are  less  toxic  than  their  lithium  counterparts.  They  are  also  easier  (less  costly)  to  obtain  in   their  anhydrous  state  and  easier   to  purify.  The  most  commonly  used  salt   is  NaClO4.  Even   it   the   salt   is   difficult   to   dry,   NaClO4,   and   in   analogy  with   the   Li-­‐ion   technology,  other   salts   used   and   practically   applicable   are   NaPF6,   NaCF3SO3,   NaFSI,   NaTFSI,   and  NaBF4,   with   the   first   being  more   common,   though   the   sensitivity   of   the   PF6   anion   to  hydrolysis  is  an  unsolved  issue.  From  a  safety  perspective,  the  larger  thermal  stability  of  sodium  salts  as  compared  to  lithium  salts  is  expected  to  be  an  advantage  [101].    Ionic  liquid  (IL)  based  electrolytes  have  also  been  considered  for  Na-­‐ion  batteries.  Most  research   has   focused   on   the   electrolyte   materials   and   not   on   battery   performance  directly.  The  main  tracks  are  related  to  the  possibilities  to  improve  the  cell  safety  based  on  the  intrinsic  properties  of  ILs.  The  conductivities,  viscosities,  and  thermal  properties  have  been  studied,  where  the  Na  electrolytes  exhibit  a  conductivity  ca  1.2  mS/cm  higher  than   the   analogous   lithium   electrolytes   [102].   Half-­‐cell   measurements   using   IL   based  electrolytes  have  been  made  as  a  proof-­‐of-­‐concept  utilising  a  binary  eutectic  of  NaFSI-­‐KFSI  exhibiting  a  conductivity  of  3.3  mS/cm  and  a  NaCrO2  electrode  [103,104].  With  an  electrochemical   stability   window   up   to   5.2   V   vs.   Na/Na+   [105]   and   a   stability   vs.   the  aluminium   current   collector,   this   type   of   electrolyte   is   clearly   interesting   for   future  applications.   The   same   electrolyte  was   also   tested   vs.   an   Sn-­‐based   alloy   anode   [106].  Utilising  an  IL  based  on  NaTFSI  in  Pyr14TFSI  has  shown  that  the  deposition  of  metallic  sodium   on   a   copper   working   electrode   does   not   occur   until   a   potential   of   -­‐0.2   V   vs.  Na/Na+,   which   thus   is   a   considerable   safety   advantage   for   work   with   low   potential  electrodes  such  as  hard  carbon  [107].  Thus  IL  electrolytes  can  act  profoundly  different,  but  still  the  Na  IL  based  electrolytes  seem  to  act  with  some  sincere  prospect  for  further  development,  especially  in  terms  of  cycling  stability.    Cell  performance:  In  2003  Valence  Technologies  reported  a  3.7  V  Na-­‐ion  cell  using  NaVPO4F  as  the  cathode  and  hard   carbon  as   the  anode   [108].  The  electrolyte  used  was  1  M  NaClO4   in  EC:DMC  and   the   cell,   tested   at   room   temperature   at   C/10,   showed   a   reversible   capacity   of   80  mAh/g,   and   the   fade  was   unfortunately   up   to   50  %   already   after   30   cycles.   Full-­‐cells  made   of   Na3V2(PO4)2F3//hard   carbon   and   an   electrolyte   of   1M   NaClO4   or   NaPF6   in  EC:PC:DMC   have   been   shown   to   operate   at   3.75   V   and   exhibit   a   theoretical   energy  density   comparable   to   that   of   graphite//LiFePO4   Li-­‐ion   cells   [99].   The   cells   tested  

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showed  a   stable   capacity  of  97  mAh/(g  cathode  active  material)   for  120  cycles  at  C/5  and  a  Coulombic  efficiency  >98.5%.      Some   recent   industrial   R&D   has   also   been   disclosed   to   some   extent,   for   example   by  Toyota.  One  openly  distributed  industrial  report  by  Sumitomo  disclosed  the  fabrication  of   NaFe0.4Mn0.3Ni0.3O2//hard   carbon   coin   and   laminated   cells   using   1   M   NaPF6   in   PC  electrolyte   exhibiting   good   cycle   life   and   rate   capability,   although  metrics   to   compare  with  similar   lithium-­‐ion  cells  were  missing  [109].  Moreover,   this  report  also  discussed  heating   and   overcharging   tests   carried   out,   indicating   a   better   performance   than   for  comparable  Li-­‐ion  cells,  as  200%  overcharge  did  only  result  in  swelling  without  burst  or  ignition.      Faradion,  a  UK-­‐based  company  developing  Na-­‐ion  cells,  claim  an  energy  density  of  their  18650-­‐cells  to  be  126  Wh/kg  and  343  Wh/L  [110],  roughly  half  of  the  Li-­‐ion  18650-­‐cells  (C//NCA   chemistry)   produced   by   Panasonic   for   Tesla   and   about   30%   more   energy  density   than   a   18650-­‐cell  made   of   C//LFP   chemistry.   Faradion   has   recently   disclosed  3Ah  Na-­‐ion   pouch   cells   using   hard   carbon   and   a   layered   oxide   cathode   (165  mAh/g)  [111].  These  are  reported  as  comparable  to  Li-­‐ion  state  of  the  art.      Even  if  the  Na-­‐ion  technology  is  still  immature,  it  is  clear  that  the  Na-­‐ion  technology  can  compete  with  the  Li-­‐ion  technology  in  several  aspects;  about  the  same  capacity  as  Li-­‐ion  materials  with  the  potential  of  lower  raw  material  costs.  With  respect  to  safety  there  is  no   indication  or  scientific  grounds  to  tell  whether  Na-­‐ion  batteries  will  be  safer  or  not  than   Li-­‐ion   batteries,   but   preliminary   accelerating   rate   calorimetry   tests   suggest   that  they  will  be  at  least  as  safe  as  Li-­‐ion  batteries  [112,113].  Aside  from  the  Na/S  and  ZEBRA  high-­‐temperature   systems   (not   treated   here),   no   commercialized   non-­‐aqueous  Na-­‐ion  cells   exist   at   present.   There   are   large   opportunities   for   research   and   development:  cathodes,  anodes,  electrolytes,  and  half  and  full  cells.      

2.4 Mg  Rechargeable  Mg  batteries  have  for  a   long  time  been  considered  as  a  highly  promising  technology.  The  theoretical  capacity  is  related  to  the  number  of  electrons  involved  in  the  redox  reactions  and  therefore  it  is  of  interest  to  use  multivalent  ions  to  double  or  even  triple  the  capacity.  Thus,  despite  their  larger  atomic  weights,  magnesium  and  aluminium  based   concepts   can   be   attractive   because   of   their   ability   to   exchange   two   and   three  electrons,   respectively,   compared   to   only   one   electron   for   lithium   and   sodium.   The  practical   capacity   in   turn  depends  on   the  amount  of   reversible   ions  during   the  charge  and  discharge  processes.  The  main  issue  is,  however,  to  find  durable  materials  for  long-­‐time  cycling  and  at  rates  needed  for  vehicle  applications.      Magnesium  possesses  several  characteristics  that  rank  it  as  one  of  the  most  favourable  metal   anodes   for   high   energy-­‐density   batteries.   Due   to   its   bivalency,   its   specific  volumetric  capacity  is  greater  than  3800  mAh/cm3,  higher  than  that  for  metallic  Li  (ca.  2050   mAh/cm3).   Moreover,   Mg   is   a   benign   and   abundant   metal   in   the   Earth's   crust.  Despite   its  potential   reactivity   it   is   stable  enough   in  ambient  atmosphere   for  handling  and  electrode  preparation  processes.  The  first  breakthrough  was  demonstrated  in  1990  with   the  development  of   an  anodically   stable  electrolyte   (an  ether   solution  containing  Mg  salts  based  on  organo-­‐borate  or  organo-­‐aluminate  anions)  [114].  A  decade  later,  the  

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next  breakthrough  was  achieved  by  the  development  of  ethereal  electrolytes  containing  Mg-­‐haloalkyl  aluminate  complexes  [115].      The   main   challenge   of   Mg   based   batteries   and   for   all   multivalent   concepts,   is   not,  however,  on  the  anode  side,  but  on  the  cathode  side.  The  cathode  requires  materials  that  allow   both   several   oxidation   state   steps   and   acceptable   diffusion   rates   of   the   Mg2+  cation.  The  ideal  material  would  be  a  compound  based  on  transition  metals  having  one  reversible  redox  couple  per  inserted  Mg2+,  i.e.  a  two-­‐electron  reduction  of  the  transition  metal,   e.g.   vanadium,   manganese,   or   titanium.   The   most   promising   materials   are  insertion  compounds  based  on  oxides  or  sulphides,  due  to  their  capacity  and  potential.      Two   main   routes   can   be   taken   to   achieve   high-­‐energy   rechargeable   magnesium  batteries:  i)  relying  on  high  capacity/low  voltage  Mg  cathodes  and  ii)  utilizing  moderate  capacity/high   voltage   Mg   ion   insertion   cathodes.   The   latter   will   be   limited   by   the  maximum   practical   intercalation   level   attainable   with   Mg   ions,   which   is   estimated   at  200–300   mAh/g.   Several   studies   have   concentrated   on   the   development   of   cathode  materials   with   higher   capacities   and   voltage   using   complex   electrolytes   [116,117].  These  cathodes  are,  however,   limited  to  about  200  mAh/g  and  a  2  V  operation  voltage  (vs.  Mg).      The  most   studied   group   of  materials   for   the   cathode   is   the  Mg-­‐based   Chevrel   phases  MgxMo6T8,   where   T   is   S,   Se,   or   a   mixture   thereof   [118].   The   structure   consists   of  octahedrally   coordinated  Mo   in   a   cubic   framework  of   the  anions  S   and/or  Se.  The  Mo  atoms   exhibit   variable   oxidation   states   and   the   anion   framework   provides   diffusion  pathways   in   several   directions   and  a   variety  of   sites   for   the   inserted  Mg2+   ions.  Up   to  four  electrons  can  be  sustained  by  the  Mo6  clusters,  resulting  in  a  theoretical  capacity  of  up   to   two  Mg2+   ions  per  MgxMo6T8  unit.  Factors  affecting   the  ability  of   the  material   to  incorporate  Mg2+   are  primarily   the   solid-­‐state  diffusion   rates,  which,   if   not   favourable  enough,  will  increase  the  electrode  polarisation  and  may  cause  ion-­‐trapping.  Secondary  factors   are   possible   co-­‐insertion   of   the   electrolyte   solvent   and   concomitant   structural  distortion   or   decomposition   of   the   positive   electrode   material.   Yet,   many   of   these  materials  suffer  from  low  electronic  conductivity  and  blended  materials  may  therefore  be   used.   The   Chevrel   phase   compounds   enable   rapid   Mg2+   diffusion   rates   due   to   the  large   amount   of   vacant   sites   available   in   the   structure   and   the   diffusion   rates   can   be  further  enhanced  at  elevated  temperatures.    The  first  successful  magnesium  battery  prototypes  used  Mo6S8  cathodes  and  were  able  to  sustain  more  than  500  cycles  at  a  moderate  rate  with  low  capacity  fading,  though  the  specific   capacity   was   rather   low   (ca.   60   mAh/g)   [115].   These   results   have   been  improved  by  substitution  of  sulphur  by  selenium,  resulting  in  increased  magnesium  ion  mobility  and  decreased  irreversible  capacity,  previously  caused  by  magnesium  trapping.  Nonetheless,  this  comes  to  the  expense  of  a  decrease  in  operation  potential  and  a  lower  electrochemical   capacity:   88.8  mAh/g   vs.   128.8  mAh/g.   A   compromise   seems   to   have  been  found  with  Mo6S8-­‐ySey  (y=1.2)  compounds  [117].    There   are   some   few   other   potential   Mg   cathode   materials   of   interest.   The   main  drawback  is  a   lower  reversible  capacity,   for  example  magnesium  cobalt  silicates  [119].  In  some  cases,  these  materials  are  targeted  to  operate  at  higher  potentials  and  enhance  the  energy  density  of  the  cells,  which  include  amongst  others,  first  principle  calculations  

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suggesting   the  suitability  of   the  MgVPO4F  compound  [120]  or  experimental  studies  on  transition   metal   oxides.   The   most   significant   reliable   findings   are   based   on   V2O5  cathodes  [121].      To  attain  a  specific  energy  comparable  to  that  of  Li-­‐ion  batteries,  further  breakthroughs  are  required  concerning  the  stability  of  the  electrolyte  towards  the  anode  (>3  V),  and  for  high  specific  energy  cathode  materials.  Any  breakthrough  in  these  directions  will  have  to   demonstrate   full   compatibility   of   the   electrolyte   with   both   electrodes,   as   well   as  allowing   fast   and   highly   reversible   magnesium   electrodeposition   and   dissolution.   In  2007  an  electrolyte  stable  close  to  3  V  was  demonstrated  [117].  The  main  bottleneck  for  any   further   development   of   high   potential   materials   for   Mg   batteries   is   indeed   the  absence  of  suitable  electrolytes  –  with  enough  stability  to  truly  test  them  in  half  or  full  cells   and   thereby   develop   a   high   potential   magnesium   based   battery   technology.  Presently   there   are   a   few   companies   trying   to   develop   rechargeable   Mg   batteries,  including  Sony,  LG  Chem,  Honda,  and  Toyota,  as  part  of  their  R&D  efforts  in  the  battery  field.   The   American   company   Pellion   Technologies   [122]   is   fully   devoted   to   the  development  of  high  energy-­‐density  Mg  rechargeable  batteries.      

2.5 Li-­S  The   reaction   of   sulphur   to   Li2S   has   a   theoretical   capacity   of   1673   mAh/g   and   in  combination  with  an  anode  of  metallic  lithium,  Li-­‐S  batteries  can  reach  gravimetric  and  volumetric  energy  densities  of  2500  Wh/kg  and  2800  Wh/L,  respectively  [123].  The  cost  of   such   a   cell  would   thus   be  much   less   than   a   corresponding   conventional   Li-­‐ion   cell  based  on  just  the  materials  cost.  The  cell  operates  in  the  voltage  range,  which  is  more  or  less  safe.  In  addition  sulphur  is  an  abundant  element  and  non-­‐toxic.  Undoubtedly,  all  of  these  advantages  make  Li-­‐S   cells   an  attractive  emerging  battery   technology.  The  main  drawbacks   are,   however,   the   insulating   nature   of   sulphur   and   polysulfide   dissolution  causing  active  sulphur  loss,  low  power  capabilities,  and  rapid  capacity  fading.      In  the  charged  states  sulphur  exists  in  the  form  of  a  large  molecule,  i.e.  S8,  in  the  cathode.  The  conversion  to  Li2S  is  a  multi-­‐step  reaction.  At  discharge  lithium  ions  from  the  anode  react  with  the  sulphur  cathode  and  long-­‐chain  lithium  polysulphides  (Li2Sx,  4≤x≤8)  are  formed   [124,125].   These   intermediate   products,   generated   at   the   initial   stages,   are  soluble  in  the  commonly  used  electrolytes.  In  the  subsequent  stages  of  discharging  these  long-­‐chain  polysulphides  will   turn   into   insoluble  Li2S2   and   finally  Li2S   [126].  A   typical  discharge  and  charge  voltage  profile  for  the  first  cycle  of  a  Li-­‐S  cell  is  shown  in  Figure  4.    

 Figure  4.  A  typical  voltage  profile  for  the  first  cycle  of  a  Li-­‐S  cell.      

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Based  on  the  reaction  steps,   the  discharge  process  can  be  divided  into  four  regions:  at  2.2-­‐2.3   V,   Li2S8   is   formed   and   dissolves   into   the   electrolyte:   S8   +   2Li   →   Li2S8,  subsequently   the   dissolved   Li2S8   transfers   to   short-­‐chain   poly-­‐sulphides   and   a  corresponding  reduction  of  the  cell  voltage  occurs;  Li2S8  +  2Li  →  Li2S8-­‐n  +  Li2Sn.  After  this  insoluble  Li2S2  or  Li2S   is   formed  at  a   second   lower  voltage  plateau  at  1.9-­‐2.1  V,  which  contributes  with  the  major  capacity  of  a  Li-­‐S  cell;  2Li2Sn  +  (2n-­‐4)Li  →  nLi2S2  or  Li2Sn  +  (2n-­‐2)Li  →  nLi2S    Hence  the  composition  of  the  electrolyte  changes  with  voltage  and  furthermore  a  redox  shuttle   mechanism   is   enabled   leading   to   that   the   theoretical   capacity   can   seldom   be  obtained   in   practice.   Besides   the   electrochemical   reactions,   complicated  disproportionation  reactions  of  the  poly-­‐sulphides  also  take  place  in  the  electrolyte;  all  affected  by  the  composition  of  the  electrolyte  and  temperature.      Based  on   above,   the  Li-­‐S   cell   is   indeed  more  of   a   liquid   electrochemical   system.   Since  both  sulphur  and  its  reduction  products  are  non-­‐conductive,  the  operation  of  a  Li-­‐S  cell  entirely   depends   on   the   dissolution   of   poly-­‐sulphides.   Reasonable   specific   capacity   of  about  800  mAh/g  in  the  first  cycle  can  remain  at  510  mAh/g  after  60  cycles  when  cycled  at  slow  rates  between  1.7  V  and  2.8  V  has  been  obtained  [127].    Lithium-­‐sulphur  batteries  have  been  studied  for  more  than  four  decades,  since  the  late  1960s   [128].   In   spite   of   the   tremendous   progress,   however,   there   are   few   reports   on  lithium   sulphur   batteries   with   appreciable   capacity   performance   up   to   1000   cycles  [129,130].   There   are   thus   still   challenges   remaining   thorny   and   unsolved.   The   first   is  associated  to  the  insulating  nature  of  sulphur  and  its  electrochemical  products  that  only  allow  ions  and  electrons  to  diffuse  on  their  surfaces.      Second,   polysulphides   as   discharge   intermediate   products   dissolve   into   the   organic  electrolyte,   which   reduces   the   amount   of   active   cathode   materials   [126,128].   The  dissolved   polysulphides   can   also   diffuse   to   the   lithium   anode   driven   by   chemical  potential   and   the   concentration   difference   between   the   cathode   and   the   anode,   be  reduced  to  Li2S  and  Li2S2,  and  deposit  on  the  lithium  anode  [131],  leading  to  undesired  parasitic   reactions.   The   last   major   problem   for   Li-­‐S   batteries   is   the   large   volume  expansion  of  sulphur  as  high  as  ~80%  during  cycling.  The  cathode  will  be  pulverised  by  the  internal  strain  resulting,  leading  to  loss  of  contact  between  the  electrode  and  current  collector  and  severe  capacity  fading.  Thus,  to  ensure  consistent  cycling  performance  of  Li-­‐S  systems  over  several  hundreds  or  thousands  of  charge/discharge  cycles  as  required  in  practical  applications,  all  these  three  major  problems  need  to  be  solved.    Despite   enormous   developments   accomplished,   the   commercialisation   of   this   battery  still  has  a  long  way  to  go,  based  on  reviewing  technological  breakthroughs.  For  vehicle  applications,   the  high  energy  density   is  attractive  both   in  terms  of  weight  and  volume.  The  high  capacity  is  also  due  to  the  involvement  of  two  electrons  in  the  redox  reactions.  The  potential  of  low-­‐cost  cells  due  to  the  abundance  of  sulphur  is  also  attractive  from  a  vehicle   perspective.   The   main   drawback   is   the   low   voltage   output   of   ca   1.8   V.  Furthermore,   the   self-­‐discharge   rate   is   high,   and   there   is   a   risk   of   H2S   evolution.   The  insulating  properties  of   the  discharge  products  can   lead   to   rapid  ageing.  Furthermore,  from  a  safety  perspective,  the  low  melting  point  of  S  at  ca.  115  °C  has  to  be  considered.      

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2.5.1 Research  trends  Most   research   efforts   are   focusing   on   development   of   novel   cathodes   via   various  synthesis   methods   in   order   to   make   different   nano-­‐structured   materials.   In   the  following   the   research   trends   for   the   cathode   and   the   electrolyte   of   Li-­‐S   cells  will   be  given,  while  the  Li  anode  is  currently  less  researched.      Cathode:  Sulphur  cathodes  for  high  capacity  and  great  cycling  stability  should  feature:  i)  sufficient  content  of  sulphur,   ii)  adequate  conductivity  (e.g.  by  adding  conductive  materials  such  as  different  kinds  of  carbon),   iii)   flexible  structure   to  buffer   the   large  volume  changes,  and  iv)  ability  to  trap  polysulphides.    Several   routes   utilising   different   nano-­‐sized   and   nano-­‐structured   sulphur   based  cathodes  are  possible,  and  the  main  research  trends  are  summarised  below.    Porous  carbon-­sulphur  composites  Micro-­‐porous  carbon  proves  to  be  an  effective  sulphur  immobilizer  because  it  has  very  small  pores   to   confine   sulphur   and  prevent   intermediate  product  poly-­‐sulphides   from  dissolve   into   the   electrolyte   [132].   Micro-­‐porous   carbon-­‐sulphur   composites   with   a  narrow  pore  size  distribution  have  shown  a   large  reversible  capacity  of  approximately  650  mAh/g   even   after   500   cycles   [133].   There   are,   however,   some   critical   factors   to  consider  for  improving  the  performance:  pore  size  and  sulphur  loading.  After  optimising  the  pore  sizes  and  sulphur  loading,  cathodes  with  an  initial  capacity  of  ca.  1400  mAh/g  and   after   100   cycles   a   capacity   retention   of   ca.   840  mAh/g   have   been   demonstrated  [134,135].   Bimodal   pore   structures   can   also   be   used   to   improve   the   performance.  Furthermore,   by   optimising   the   initial   ratios   of   carbon/silica/surfactant,   cathode  materials   have   been   demonstrated   having   an   initial   capacity   of   995   mAh/g   and   a  capacity  of  550  mAh/g  after  100  cycles  at  1  C  rate  [132,136].    Sulphur  containing  nano-­tubes/nano-­fibres    Nano-­‐tubes   and   nano-­‐fibres   of   carbon   have   been   shown   to   be   attractive  matrices   for  sulphur.   The  main   reason   is   the   intimate   contact   between   conductive   nano-­‐tubes   and  sulphur   enabling   fast   electron   and   ion   transport   in   electrodes   [137].   In   addition,   the  nano-­‐tubes   can   accommodate   the   volume   expansion   of   sulphur   during   cycling.   One  example   is  cathodes  made  by  CVD  deposition  of  carbon  nano-­‐tubes  filled  with  sulphur  delivering  a  capacity  of  more  than  1400  mAh/g  [138].  This  cathode  not  only  facilitates  the  transportation  of  electrons  and   ions,  but  also  shortens  the  distance  of   lithium  ions  diffusion  in  electrodes,  contributing  to  fast  kinetics.  Moreover,  polymer  materials  can  be  used  mainly  due  to  strong  physical  bonds  and  chemical  interactions  among  the  polymer  framework,  sulphur,  and  poly-­‐sulphides.  Examples  of  capacities  are  837  mAh/g  at  0.1  C  after  100  cycles  and  432  mAh/g  at  1  C  after  500  cycles  [139].      Graphene-­sulphur  composites    Despite   the   high   conductivity,  mechanical   strength,   and   flexible   structure   graphene   is  not  widely  investigated  as  the  host  for  sulphur  in  the  cathode  due  to  its  sheet-­‐like  shape  and  open  structure,  causing  active  materials  to  readily  diffuse  out.  There  are,  however,  a  few  attempts  to  use  graphene  in  Li-­‐S  cells,  [140,141],  for  example  by  sandwich  sulphur  between  two  graphene  layers  [142]  or  by  coating  sulphur  particles  with  a  polymer  and  then   wrapping   these   coated   particles   with   graphene   [143].   The   cathodes   made   have  

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displayed  reversible  capacity  of  ca.  600  mAh/g  and  less  than  15%  degradation  after  100  cycles.    3D  nano-­structured  sulphur  composites    3D  nano-­‐architectures  provide  porosity   for   increased  power  by   reducing   the  diffusion  lengths   and   can   accommodate   volume   changes   during   cycling.   Also,   porosity   offers  necessary   pathway   for   electrolyte   penetration,   resulting   in   enhanced   lithium   ion  diffusion   and   fast   kinetics.   Thus,   considerable   efforts   have   been   devoted   to   the  development   of   3D   electrodes   in   order   to   achieve   high   energy   density   and   high   rate  capability   Li-­‐S   batteries   [137,144,145].   3D  multi-­‐walled   carbon   nano-­‐tubes   have   been  developed   to  house   sulphur  or   lithium   sulphide   and   achieved   enhanced   capacity  with  780  mAh/g  remaining  after  200  cycles  at  a  current  density  of  0.5  A/g  [146].  3D  polymer  nano-­‐tubes  have  been  investigated  as  well  and  compared  to  carbon  nano-­‐tubes,  polymer  nano-­‐tubes  enable  trapping  of  intermediate  poly-­‐sulphides  effectively,  rendering  better  cell  performance  [147,148].  Cathodes  of  a  high  sulphur  loading  (70  wt  %)  have  shown  a  reversible  capacity  of  ca.  500  mAh/g  at  1  C  rate    [140].    Core/yolk-­shell  structures    Core-­‐shell  structures  are  attractive  addressing  the  challenges  of  volume  changes  during  cycling   and  preventing   poly-­‐sulphide   dissolution.   Yolk-­‐shell   structures   have   also   been  investigated,   showing  promising   results   [149].   The  main  difference  between   the   yolk-­‐shell   structure  and  core-­‐shell   structure   is   that  void  space  exists  between   the  core  and  the   shell   in   the  yolk-­‐shell   structure.   In  most   core-­‐shell   structures,   sulphur  or   sulphur-­‐based  compounds  act  as  the  core  and  high  sulphur  contents  (up  to  ca.  85%)  embedded  in   the   shell  have  been   reported   [150,151].  The   shell  provides  protection  against  poly-­‐sulphide  dissolution  and   is  usually  conductive  materials  able   to   facilitate  both   ion  and  electron  transport.  Although  most  work   focuses  on  sulphur  as   the  core  material,  some  research  has  been  done  using  sulphur  as  the  shell,  however,  resulting  in  poor  capacity  retention   [152].   The   coatings   can   be   attributed   to   lower   capacities   mainly   due   to   i)  insufficient   coating  with   conductive  materials,   resulting   in  polysulfide  dissolution   into  the  electrolytes,   and   ii)   the  volume  expansion  and  constriction  during  electrochemical  processes   leading   to   fracture   of   the   shell   structure   and   leakage   and   dissolution   of  polysulphides.  There  are  two  general  approaches  that  can  be  considered  to  address  the  aforementioned   tricky   problems.   The   first   is   to  modify   the   sulphur   core   by   using   for  example   smaller   sulphur   allotropes   [153]   or   ultrafine   sulphur   [154].   The   second  approach  is  to  modify  the  core-­‐shell  framework  by  for  example  making  double  shells  to  increase   the   effectiveness   of   preventing   polysulfide   dissolution,   making   soft   shells   to  accommodate   volumetric   expansion   in   discharge,   utilising   yolk-­‐shell   structures,   or  combinations  thereof.      Li2S  cathodes  The   final   discharge   product   of   sulphur   electrodes,   Li2S,   has   been   investigated   as   a  potential  cathode  material   for  high  energy  Li-­‐S  cells   [e.g.  155].  Moreover,  compared  to  the  sulphur  cathode,   the  Li2S  cathode  material   is  a  pre-­‐lithiated  material  not  requiring  lithium  metal  as   the  anode,  mitigating  safety  concerns  caused  by   lithium  metal  and   its  dendrites.  Li2S  can  be  paired  with  other  promising  anodes,  such  as  silicon  [156]  and  tin  [157].  A  big  challenge  for  Li2S  cathodes  are  the  slow  kinetics  and  low  rate  capability  as  a  result  of  the  poor  electronic  and  ionic  conductivities  of  Li2S.  The  high  dissolution  of  poly-­‐sulphides   is   also   a   concern.   Different   carbon-­‐coated   Li2S   materials   have   also   been  

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utilized   directly   as   the   cathodes   [e.g.   157].   The   main   concern   is   the   high   sensitivity  towards  water,  putting  high  restrictions  on  the  production  process.    Anode:  The  most  common  anode  used  is  metallic  lithium  and  has  been  described  previously.  In  the  special  environment  in  the  Li-­‐S  cell  the  metallic  lithium  anode  needs  to  be  protected  to  prevent  redox  shuttle  mechanisms,  to  reduce  gassing  (swelling),  and  to  increase  the  safety.  In  order  to  protect  Li  effectively,  the  material  of  the  protective  layer  should  be:  i)  insoluble   in   the   liquid   electrolyte,   ii)   chemically   stable   against   poly-­‐sulphides   and  metallic   Li,   and   iii)   highly   ionic   conductive.   The   protection   can   mainly   be   of   the  characters:  physical  barrier,  using  a  gel  polymer  electrolyte  or   lithium  alloys  as  anode,  or   the  metallic   lithium  can  be  pre-­‐passivated  before   cell   assembly.  The  main   issue   for  these   protective   layers   is   their   ability   to   not   hamper   the   power   capabilities   of   the  anodes.    A  separator  can  be  utilised,  and  one  example   is   to  use   fluorinated  polymers   forming  a  LiF  passivation  layer  on  the  surface  of  Li  anode  through  a  limited  reaction  between  the  Li  metal  and  polymer  [158].  This  approach  has  proven  to  improve  the  cycling  efficiency  and   morphology   of   Li   metal   although   the   concern   with   the   chemical   stability   of  fluorinated  polymers  against  the  poly-­‐sulphides  still  remains.  Moreover,  it  is  possible  to  use   organosulphur   compounds,  which   can   form   complex  with   Li  metal   and   thereby   a  protective  layer  [159].      A  gel  polymer  electrolyte  able  to  stick  the  separator  and  Li  anode  together  can  be  used,  which   helps   to   improve   the  morphology   of   Li   plating   by   forming   a   layer   between   the  separator  and  Li  anode.  The  capacity  retention  have  shown  to  be  improved  and  the  on-­‐set   temperature   for   thermal   runaway   increased   by   at   least   50   °C   due   to   the   compact  deposition  of   the  Li  metal  [160].  Furthermore,  a  Li-­‐Al  alloy  by   laminating  a  thin  Al   foil  with  a  Li  anode  has  been  proposed  to  reduce  the  redox  shuttle  mechanism  and  has  been  shown  to  improve  the  specific  capacity  and  capacity  retention  [161].  A  similar  concept  has  been  demonstrated  using  Pt   and  a  Li-­‐S   cell  with  a   specific   capacity  of  750  mAh/g  after   90   cycles   has   been   demonstrated   [162].   Pre-­‐treatment   of   the   Li   anode   using   a  reactive  chemical  to  form  a  stable  passivation  layer  before  the  Li-­‐S  cell  is  assembled  has  been   investigated   by   using   oxidative   compounds   and   inorganic   acids   to   form   the  insoluble   and   stable   protective   layer   on   the   Li   metal.   A   Li-­‐S   cell   with   pre-­‐treated   Li  showed  higher  discharge  capacity  as  compared  with  the  baseline  cell  [163].    Electrolytes:  Considerable   efforts   to   solve   poly-­‐sulphide   dissolution   and   the   shuttle   phenomenon  have  been  paid  to  electrolyte  studies.  Developing  new  electrolytes  is  extremely  desirable  for  realising  good  interfacial  architectures  and  great  properties  of  Li-­‐S  batteries.  In  the  following   the   main   research   trends   concerning   electrolytes   for   Li-­‐S   cells   are  summarised.      Liquid  electrolytes  Properties   for   electrolytes   applicable   for   Li-­‐S   cells   include   good   polysulfide   solubility,  chemical   stability   towards  polysulfide  species   (anions  and  anionic   radicals)  and   the  Li  anode,  and   low  viscosity   for   fast   ion  and  charge   transport.  Based  on   the  requirements  above,  conventional  carbonate  solvents  used   in  Li-­‐ion  cells  are  usually  not  suitable   for  

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Li-­‐S   cells   due   to   their   chemically   reactivity   with   poly-­‐sulphides   [164,165].   Therefore,  alternative   ether   solvents   and   poly(ethylene   glycol)   are   considered     [166,167].   It   is  found   that   cyclic   and   linear   ethers,   including   tetrahydrofuran,   1,3-­‐dioxolane,   1,2-­‐dimethoxyethane,   and   tetra(ethylene   glycol)   dimethyl   ether,   are   suitable   electrolytes,  where   cells   based   on   the   latter   has   shown   high   capacities   of   over   1200  mAh/g   [168-­‐170].      Solid  electrolytes  The  shuttle  phenomenon  is  inevitable  when  liquid  electrolytes  are  employed  in  Li-­‐S  cells  due   to   poly-­‐sulphide   dissolution   and   reduction   on   the   anode   surface.   In   order   to  eliminate   the   existence   of   poly-­‐sulphide   ions,   solid   electrolytes   have   attracted   intense  attention  as  an  alternative  approach.  The  requirements  of  solid  electrolytes  for  Li-­‐S  cells  include   good   Li-­‐ion   conductivity,   high   stability   towards   the   lithium  metal   anode,   and  high  contact  area  between  electrodes  and  electrolytes.  The  main  hurdle  for  the  practical  application   of   solid   electrolytes   is   their   low   ionic   conductivity.   However,   with   the  emergence   of   solid   media   possessing   lithium   ion   conductivity   comparable   to   that   of  liquid   electrolytes   [171],   all-­‐solid   state   electrolyte   cells   could   become   the   next-­‐generation  batteries  (as  described  previously).  Since  the  ion  conductivity  is  challenging  for   all-­‐solid-­‐state   Li-­‐S   cells,   PEO   with   lithium   salts   containing   finely   dispersed   nano-­‐sized  ZrO2  particles   or   LiAlO2   filler   has   been  developed   [172,173].  A   capacity   close   to  theoretical   and   high   Coulombic   efficiency   are   both   achieved   only   at   elevated  temperatures   (90   °C),   and   a   stable   anode   interface   is   obtained,   likely   due   to   the  dispersed   ceramic   filler   as   an   interfacial   stabiliser   [172].   Also,   various   other   solid  electrolytes,   such   as   Li2SeSiS2   powers,   thio-­‐LiSICONs   (lithium   super-­‐ionic   conductor),  and  Li2SeP2S5  glass-­‐ceramics  [e.g.  174,175],  have  been  investigated.      Gel  polymer  electrolytes  Theoretically,   gel   polymer   electrolytes   (GPE)   have   advantages   of   solid   electrolytes  impermeable   to  poly-­‐sulphides  and  suppressing  dendrite   formation  and  advantages  of  liquid  electrolytes  with  good  conductivity.  One  example   is  PEO/LiCF3SO3,  EC/DMC  and  LiPF6,   resulting   in   high   interfacial   resistance   and   slow   capacity   retention   [157].   A  favourable   alternative   to   have   higher   capacity   retention   is   to   use   electro-­‐spun   nano-­‐fibrous  membranes  based  on  different   polymers   [176].   These  GPE  not   only  have  high  interfacial   compatibility,  wide   oxidation   stability   and   high   ionic   conductivity,   but   also  possess   high   liquid   electrolyte   uptake   and   serve   as   a   good   host   due   to   a   fully  interconnected   pore   structure   of   polymer   membranes   [177].   Another   approach   to  enhance  the   ionic  conductivity  and  maintain  the   liquid  electrolyte  within  the  GPE  is  to  use   functional   groups   to   bond   with   the   liquid   electrolyte;   for   example   functionalised  poly(methyl-­‐methacrylate)  (PMMA)  containing  trimethoxysilane  domains  blended  with  PVDF-­‐HFP   [178].   Li-­‐S   cells   with   this   GPE   have   delivered   higher   ionic   conductivity  compared   to   that   of   GPE   without   functionalized   groups,   and   displayed   little   capacity  decay.    Ionic  liquid  electrolytes  In   addition   to   the   three   types   of   electrolytes   discussed   above,   ionic   liquid   (IL)  electrolytes   have   also   attracted   much   interest.   Ionic   liquids   are   defined   as   liquid  comprising   entirely   ions.   They   have   a   variety   of   merits,   including   negligible   vapour  pressure,  non-­‐flammability,  high  lithium  ion  conductivity,  wide  electrochemical  stability,  and  the  ability  to  inhibit  the  formation  of  lithium  dendrites  [e.g.  179].  IL  electrolytes  can  

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enhance   the   performance   of   sulphur   cathodes:   for   example   EMITFSI   [180]   P1A3TFSI  [179],  have  been  investigated  as  the  electrolytes  for  Li-­‐S  cells.  Moreover,  IL  can  suppress  the   solubility   of   Li2Sx   (2≤x≤8)   in   the   IL,   whereas   the   dissolution   and   precipitation   of  Li2Sx  take  place  in  the  organic  electrolyte  [181].  However,  considering  the  viscosity  and  high  price  of  IL,  it  is  not  cost-­‐effective  to  totally  employ  IL  as  the  solvents  of  electrolytes,  and  combinations  of  IL  and  organic  solvents  are  investigated  [165,182].  For  example  the  viscosity   and   conductivity   of   electrolytes   can   be   adjusted   with   different   contents   of  PYR14TFSI,   and   have   shown   that   the   shuttle   mechanism   is   greatly   inhibited   and   a  Coulombic   efficiency   over   98%  upon  100   cycles   can   be   obtained  when   the   content   of  PYR14TFSI   is   50   vol.%   [165,182].   It   should   be   noted   that   the   SEI   exhibits   different  morphology  when   ionic   liquids   are   used   including  more   stable   properties   against   the  corrosion  of  poly-­‐sulphides  [183].      

2.6 Li-­oxygen  Lithium   air   or,   more   accurately,   Li-­‐oxygen   (Li-­‐O2)   batteries   have   been   particularly  tantalizing   because   of   their   very   high   gravimetric   theoretical   energy   densities   (11-­‐13  kWh/kg).  These  numbers  are,  however,  misleading  and  come  from  a  simple  calculation  of  how  lithium  metal  reacts  electrochemically,  and  the  reaction  products  are  not  taken  into   account.   Accounting   for   Li2O2   formation   the   theoretical   energy   density   drops   to  about  3500  Wh/kg,  and  practical  values  on  the  system  level  will  be  even   lower  (likely  less  than  300  Wh/kg).        Currently,   four   types   of   Li-­‐O2   cells   are   under   development   and   are   designated   by   the  type  of  electrolyte  employed:  aprotic,  aqueous,  solid-­‐state,  and  hybrid  aqueous/aprotic.  For  all  types  of  Li-­‐O2  cells,  an  open  system  is  required  to  obtain  oxygen  (from  the  air),  as  oxygen  is   the  active  material  of   the  cathode.  Li  metal  must  be  used  as  the  electrode  to  provide  the  lithium  source  for  all  the  systems  at  the  current  stage.  In  aprotic  Li-­‐O2  cells,  porous   carbons   are   used   as   the   reservoir   for   the   insoluble   discharge   products,  presumably   Li2O2.   In  most   cases,   electrocatalysts   are   essential   to   promote   the   oxygen  reduction   and   oxygen   evolution   reactions   during   the   cell   discharge   and   charge  processes.   In   the   following   focus   is   on   the   aprotic   Li-­‐O2   cells,   since   it   is   the  dominant  concept  for  research  efforts  and  presently  being  the  most  mature.  A  typical  aprotic  Li-­‐O2  cell   composes   a   metallic   lithium   anode,   a   non-­‐aqueous   electrolyte,   and   a   porous   O2-­‐breathing   cathode   that   contains   carbon   particles   and,   in   most   cases,   an   added  electrocatalyst.   It  should  be  noted  that  the  oxygen  reduction  reaction  during  discharge  and   oxygen   evolution   reaction   during   charge   of   a   Li-­‐O2   cell   occur   at   a   three-­‐phase  boundary  involving  the  solid  electrode,  liquid  electrolyte,  and  oxygen  gas,  which  makes  the  Li-­‐O2  cell  more  complicated  than  the  conventional  Li-­‐ion  cell  (more  resembling  a  fuel  cell).    During   discharge   oxygen   is   reduced   at   the   cathode   and   combines   with   lithium   ions  supplied   from   the   anode   to   form   Li2O2   at   a   voltage   of   about   3   V   vs.   Li/Li+.   Practical  discharge   voltages   range   from   ca.   2.5-­‐2.8   V   vs.   Li/Li+   [184-­‐190]   and   therefore   the  gravimetric   energy   advantage   compared   to   Li-­‐ion   cells   arises   from   the   significantly  larger  gravimetric  capacities  attainable  with  the  non-­‐intercalation  O2  electrodes.  Energy  densities   of   Li-­‐O2   cathodes   in   the   discharged   state   (considering   the  weight   of   carbon,  catalyst,   the   Li2O2   formed)   have   been   obtained   in   the   range   of   1800-­‐2800   Wh/kg,  depending   on   cathode  material   used   [184-­‐188],   corresponding   to   ca.   55-­‐85  %   of   the  

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theoretical   upper   limit.   These   energy   densities,   which   conservatively   consider   the  performance  when  the  electrode  is  in  the  discharged  (heaviest)  state,  represent  roughly  a  3-­‐5   times   improvements   compared   to  Li-­‐ion   cells  with  electrode  energies  of   ca.  600  Wh/kg   at   comparable   low   power   density   (50   W/kg)   [191].   When   comparing   the  capacity  and  energy  density   for  Li-­‐O2  cells  with  other  battery   technologies,   the  weight  and  volume  of  the  oxygen  or  the  reaction  product  (Li2O2)  must  be  included  to  make  the  comparison  of  results  fair.        There   are   many   challenges   to   make   rechargeable   Li-­‐O2   cells   attainable   for   practical  usage.  The  round-­‐trip  efficiency  of  Li-­‐O2  cells  with  carbon  cathodes  without  catalysts  is  below  70%  [187,192],  significantly  lower  than  the  round-­‐trip  efficiency  of  conventional  Li-­‐ion  cells,  often  at  85-­‐95%  [193].  Moreover,  the  current  densities  demonstrated  of  Li-­‐O2   cells   are   in   the   range   of   0.1-­‐1  mA/cm2   [186,194],   which   is   about   10   to   100   times  lower  than  that  of  Li-­‐ion  cells  (ca.  30  mA/cm2)  [194].  The  main  reason  for  the  low  rate  capability  is  related  to  oxygen.  There  is  a  practical  limit  how  fast  oxygen  can  be  brought  into  the  cell,  which  in  turn  diminishes  the  reaction  rates.  Most  cells  produced  so  far  are  small  laboratory-­‐scale  cells  where  limited  oxygen  flow  is  not  yet  an  issue,  as  it  will  be  for  large  automotive  cells.  This  will   result   in   the  need  of  more  cells   in  parallel   in  order   to  achieve   the  same  performance,  adding  more  cost,  weight,   and  volume  of   the  complete  battery  pack  for  vehicle   installation.  Furthermore,   the  cycle   life  of  Li-­‐O2  cells  shown  to  date  is  up  to  100  cycles  [195,196],  which  is  significantly   lower  than  that  of  Li-­‐ion  cells  (up  to  5000  cycles)  [197].  These  technological  challenges  are  strongly  related  to  various  scientific  challenges  of  the  materials,  including  the  chemical  instabilities  and  the  lack  of  fundamental  understanding  of  the  reaction  and  transport  kinetics.  Moreover,  the  safety  characteristics  of  the  anode  and  the  sensitivity  for  contaminations  from  H2O  and  air  are  still  to  be  understood.  A  major  challenge  to  moving  ahead  even  at  the  research  level  is  to  find   a   stable   electrolyte   for   the   oxygen   electrode.   The   research   efforts   to   tackle   these  challenges  are  summarised  in  the  following.    

2.6.1 Research  trends  Electrolyte:  The  organic  electrolytes  (both  solvents  and  lithium  salts)  play  the  most  critical  role  in  an  aprotic  Li-­‐O2  cell  and  determine  whether  a  truly  rechargeable  Li-­‐O2  cell  can  be  realised.  Numerous   studies   have   shown   that   the   stability   of   the   electrolytes   during   the   oxygen  reduction  and  oxygen  evolution  processes  is  the  key  challenge  for  the  aprotic  Li-­‐O2  cell.  With  no  doubt,  searching  for  fully  stable  electrolytes  in  the  oxygen-­‐rich  including  super-­‐oxide   and   peroxide   electrochemical   environment   is   the   research   priority   at   present.  Unfortunately,   no   single   electrolyte   investigated   so   far   meets   these   demanding  requirements,   despite   extensive   efforts   in   the   past   few   years.   Understanding   the  reaction  mechanisms  between  the  electrolytes  and  active  oxygen  reduced  species  will,  no   doubt,   be   the   key   to   develop   a   stable   electrolyte   for   Li-­‐O2   cells   [198].   Carbonate-­‐based   electrolytes   have   proved   to   be   highly   unstable   towards   the   oxygen   reduction  species.  However,  there  is  still  a  large  amount  of  research  work  using  carbonate-­‐based  electrolytes   to   investigate   the   catalytic   activities   of   the   cathode   materials   [199-­‐201],  despite  the  fact  that  the  severe  instability  of  these  electrolytes  has  been  reported.  Ether-­‐based   electrolytes   have   been   shown   to   be   relatively   stable   in   the   presence   of   the  reduced  oxygen  species;  the  most  promising  solvents  are  DMSO  and  TEGDME  [202-­‐206].  

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Their  electrochemical  behaviour  and  durability  remain  to  be  investigated.  The  research  direction  is  towards  mixed-­‐solvent  electrolytes  [207],  as  for  Li-­‐ion  cells.    Also  the  lithium  salt  used  has  an  effect  on  the  stability  of  the  electrolytes  in  Li-­‐O2  cells.  For   example,   lithium   hexafluorophosphate   (LiPF6),   which   is   used   in   most  commercialised  Li-­‐ion  cells,  has  been  shown  to  react  with  Li2O2  [208,209]  and  the  nickel  foam   current   collector   could   be   oxidised   at   a   potential   beyond   3.5   V   vs.   Li/Li+   [210].  Several   Li-­‐salts   (LiBF4,   LiPF6,   LiClO4,   and   LiTFSI)   have   been   investigated   in   terms   of  anion  stability.  LiClO4  has  shown  to  be  the  least  reactive  towards  O2-­‐  radicals  [211],  but  it   is  unstable   in  an  O2  rich  environment   [212].  Lithium  bis(oxalato)borate  (LiBOB)  has  also  been  investigated  as  a  potential  salt  [213].  The  BOB-­‐  anion  reacted,  however,  with  the  O2-­‐  radicals   to   form  LiB3O5.  Moreover,   the  anions  react  with  the  current  collectors,  such  as   the  widely  employed  Al   foils.  LiTFSI,  LiC(CF3SO2)3,  and  LiCF3SO3  have  all  been  reported  to  react  with  Al  [214,215].  Therefore,  Al  foils  should  be  avoided  and  replaced  by  other  current  collectors.  Although  limited  attention  has  been  paid  to  Li-­‐salts  in  Li-­‐O2  cells,   the   possible   decomposition   and   accordingly   the   effect   of   the   anions   on   the  performance  of  Li-­‐O2  batteries  should  not  be  neglected.    Electrolyte   additives   can   improve   the   dissolution   of   Li2O2   and   O2   in   electrolytes   and  increase   the   discharge   capacity   of   the   cell.   One   example   is   to   add  tris(pentafluorophenyl)borane   to   carbonate-­‐based   electrolytes   to   increase   the  dissolution  of  the  discharge  product  Li2O2  and  enable  further  oxygen  reduction  reaction  to   occur   at   the   released   active   sites   [216].   To   increase   the   O2   solubility   in   PC-­‐based  electrolytes   per-­‐fluorotributylamine   has   been   used   [217,218].   The   idea   of   storing   the  discharge   product   Li2O2   in   the   electrolyte   is   promising   for   improving   the   specific  capacity  and  avoiding   the  blockage  of   channels   in   the  cathode,  but   the  stability  of   this  system  needs  careful  evaluation.    Cathode:  One   of   the   largest   hurdles   for   the   rechargeable   Li-­‐O2   cells   is   the   large   overpotential  during  discharge  and  charge  (about  1  V),  even  at  very  low  current  density  (0.01−  0.05  mA/cm2),   which   results   in   low   round-­‐trip   efficiencies   and   low   power   capability.   This  limitation  is  strongly  believed  to  depend  on  the  nature  of  the  catalytic  properties  of  the  cathode,   in  addition   to   the  stability  of   the  electrolytes.  Many  catalysts,   including  metal  oxides,   non-­‐precious,   and   precious   metals   on   a   porous   carbon   support,   have   been  examined   as   parts   of   the   cathode   material   for   the   oxygen   reduction   and   evolution  reactions,   showing   large  differences   in  discharge   capacity  among  different   catalysts.  A  nearly   identical   discharge   voltage   plateau   at   about   2.6-­‐2.7   V   is   observed   for   different  catalysts,  similar  to  that  for  the  bare  carbon  without  any  catalyst  loading.  This  probably  implies  that  either  the  oxygen  reduction  reaction  kinetics   in  cathodes   is   limited  by  the  oxygen   mass   transport   toward   the   catalysts   or   carbon   itself   can   provide   sufficient  electrochemical   activity.   Understanding   the   role   of   carbon   in   the   electrochemical  reactions  in  the  Li-­‐O2  cell  could  provide  guidance  and  a  baseline  for  identifying  efficient  catalysts  and  to  improve  the  cell  performance.    Porous  carbon  is  the  most  commonly  used  cathode  material,  mainly  due  to  the  sufficient  charge   transfer   for   the   electrochemical   reactions   and   space   for   housing   the   discharge  products.  Moreover,  the  low  mass  of  the  carbon-­‐based  electrode  results  in  high  specific  capacities.   Different   types   of   carbon   have   shown   certain   catalytic   activity   towards  

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oxygen  reduction  [219-­‐221].  Aside  from  the  commercially  available  carbon  black,  recent  studies   have   shown   that   other   carbon-­‐based   materials   could   also   be   very   successful  when   used   with   a   stable   electrolyte,   for   example   hollow   carbon   fibres   showing   an  energy  density   about   four   times   that  of   a  LiCoO2   cathode   for  Li-­‐ion   cells   (ca.   2500  vs.  600   Wh/kg)   [222].   This   was   due   mainly   to   low   carbon   packing   and   highly   efficient  utilisation   of   the   available   carbon   mass   and   void   volume   for   Li2O2   formation.   The  visualisation   of   Li2O2   morphologies   upon   discharge   and   disappearance   upon   charge  represents  a  critical  step  towards  understanding  of  the  key  processes  that  limit  the  rate  capability  and  low  round-­‐trip  efficiencies  of  Li-­‐O2  cells.  The  morphology  of  Li2O2  leads  to  very   non-­‐uniform   surface   coverage,   which   is   beneficial   to   increasing   the   discharge  capacity   of   the   cell   due   to   easier   oxygen   diffusion   at   the   late   stage   of   the   discharge.  Therefore,  understanding  and  controlling  the  nucleation  and  morphological  evolution  of  Li2O2   particles   upon   discharge   is   the   key   factor   to   achieving   high   volumetric   energy  density   of   the   Li-­‐O2   cell   [219,223].   Graphene-­‐based   materials   have   also   been  investigated  due  to  their  low  weight,  high  conductivity,  and  catalytically  active  surface.  High  electrode  capacities  have  been  reported  based  on  graphene  (about  15000  mAh/g  graphene)  [224].  It  should  be  noted,  however,  that  this  capacity  has  been  obtained  in  a  primary   cell   containing  porous   functionalised   graphene   sheets.   The   Li2O2  morphology  (e.g.   shape   and   thickness)   and   structure   (e.g.   crystallinity   and   surface   vs.   bulk  composition)  are   important  parameters   that  can   influence   the  discharge  capacity,   rate  capability,   and   cyclability   of   Li-­‐O2   cells   and   is   critical   for   identification   of   new  approaches  to  reduce  the  overpotentials  during  cycling  [225].    Moving   on   to   catalysts,   various   catalysts   have   been   examined,   such   as   metal   oxide,  precious  metals,  and  non-­‐precious  metals,  and  the  most  studied  is  MnO2  [220,226,227].  The   crystal   structure   (e.g.   α,   β,   δ,   γ,   and   λ)   and   the   morphology   of   the   MnO2   nano-­‐particles  can  be  tailored  to  achieve  different  properties  and,  thus  catalytic  performance  in  Li-­‐O2  cells:  nano-­‐sheet  δ-­‐MnO2  microflowers,  α-­‐MnO2  nano-­‐wires,  and  α-­‐MnO2  nano-­‐tubes   [228].   In   terms   of   the   electrocatalytic   activity   for   these   different   MnO2   nano-­‐particles,   the   α-­‐MnO2   nano-­‐tubes   exhibit   much   better   performance   to   catalyze   the  electrochemical  processes   in  aprotic  Li-­‐O2   cells.  As-­‐prepared  MnO2/C  composites  with  porous  structures  and  high  specific  surface  area  provide  more  active  sites  for  the  oxygen  reduction  and  evolution  reactions  and,  therefore,  lead  to  significant  enhancement  of  the  electrochemical  performance  of  Li-­‐O2  cells,  and  hence  a  lower  charge  overpotential  (3.5  V)  could  be  achieved  [220].  The  use  of  carbon  supported  metal  oxides  as  catalysts  for  Li-­‐O2  cells  was  initially  motivated  by  studies  in  aqueous  Zn–air  or  fuel  cell  systems,  where  MnO2,  Co3O4,  LaNiO3-­‐x,  and  Pb2Ru2O7-­‐x  oxides  have  shown  significant  catalytic  activities  for  oxygen  [229,230].      Carbon  supported  precious  metal   catalysts  have  been  extensively   studied   in   fuel   cells,  and  they  have  now  also  been  investigated  for  Li-­‐O2  cells.  Pt  and  Pd  supported  on  carbon  has   shown   to   be   functionable   [231,232]   and   carbon   supported   Ru   could   significantly  increase   the   kinetics   of   Li2O2   decomposition   on   charge   [233].   The   possible   catalytic  activity  towards  electrolyte  decomposition  has  to  be  taken  into  account,  also  for  Pt  and  Pd   [234,235].  One  example  of   a  non-­‐oxide,  non-­‐precious   catalyst   is  Fe–N–C  which  has  been  demonstrated  to  have  catalytic  activity  towards  the  reduction  of  oxygen  in  aqueous  electrolytes   [235,236].   The   cell   voltage   was   lower,   however,   compared   to   an   MnO2-­‐based  catalyst,  but  a  reduction  in  the  overpotential  of  ca.  0.6  V  was  observed,  which  is  promising.  Moreover,   only   O2  was   released   upon   charge;   for   the  MnO2-­‐based   catalyst  

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also   CO2   was   released,   and   a   suggested   reason   proposed   is   an   increased   interfacial  contact  with   lithium  peroxide,  which  could  efficiently   lower   the  activation  barrier  and  reduce  the  overpotential  during  charge  [237].    Anode:  Currently,   Li   metal   is   used   as   anode   and   will   possibly   be   replaced   by   another   large  capacity  materials  for  the  sake  of  safety  before  deployment,  if  the  metallic  lithium  anode  cannot   be   stabilised.   This   has   been   described   previously   and   will   therefore   not   be  further   discussed   here.   From   the   known   anode   materials   for   Li-­‐ion   cells,   Si   and   its  related  materials   are   the  most  promising   alternatives   [238].   The  difficulty   is   to  buffer  the  volume  change  and  reduce  the  pulverisation  of  Si  particles  during  cycling.  Assuming  a  discharge  voltage  of  2.4  V,  the  energy  density  of  the  LixSi-­‐O2  cell  has  been  estimated  to  980  Wh/kg,  considerably  higher  than  the  384  Wh/kg  offered  by  conventional  3.6  V  Li-­‐ion  graphite//LiCoO2  cells  [238].    The  reactions  occurring  on  the  interface  between  the  Li  electrode  and  electrolyte  in  Li-­‐O2  cells  are  complicated  due  to  O2  crossover  from  the  cathode.  For  this  reason,  we  only  discuss   the   effect   of   oxygen   crossover   on   the   degradation   of   the   Li   electrode   and   the  possible  solutions  to  improve  the  Li  electrode  performance  in  Li-­‐O2  cells.  As  for  all  other  anodes  having  a  potential   lower  than  the  HOMO  level  of  the  electrolyte,  the  electrolyte  will  decompose  at   the  anode   forming  various  compounds  –   the  SEI   layer  –  which  may  block  the  Li-­‐ion  diffusion  leading  to  poor  cell  performance.  Controlling  the  SEI  reactions  at  the  lithium  electrode  through  suitable  membranes  or  passivation  films  is  essential  for  achieving  good  performance  of  Li-­‐O2  cells.  Previous  studies  have  suggested  the  need  for  an   efficient   protective   layer   for   metallic   lithium   to   avoid   decomposition   of   the   Li  electrode   due   to   contamination   by   discharge/charge   products   in   the   Li-­‐O2   cells   [239-­‐242].    The   separators   currently   used   in   Li-­‐O2   cells   cannot   prevent   O2/H2O   diffusion   to   the  metallic   Li   anode,   and   oxygen   crossover   can   be   a   significant   problem,   leading   to   fast  decay   of   the   Li   electrode.   It   is   critical   to   develop   thin,   active  membranes   that   can   be  embedded  within  passive  porous  polymeric  membranes  to  effectively  eliminate   the  O2  crossover,  and  thereby   increasing  the  reversibility  of  Li-­‐O2  cells.  To  address   this   issue,  studies  have  been  performed  on  Li-­‐ion  conducting  membranes.  One  example  is  to  utilise  Li+   conducting   Si-­‐membranes   [241].   Compared   with   the   NASICON-­‐type   lithium   ion  conducting  membranes,  the  Li+  conductivity  of  these  Si-­‐membranes  is,  however,  still  too  low  (3-­‐4   times   lower).  Research  efforts   towards  protective  and  stabilising  membranes  are  expected  to  grow.    

2.6.2 System  implications  Packaged  Li-­‐O2  cell  prototypes  have  not  been  widely  developed  and  the  true  gravimetric  energy   advantage   of   devices   is   not   known,   although   it   is   expected   to   be   significantly  lower   than   the   theoretical   values.  A   calculation  of   the  energy  densities   for  a  Li-­‐O2   cell  (Li2O2  as  the  final  reaction  product)  and  a  comparison  with  a  graphite//LiCoO2  based  Li-­‐ion  cell  has  been  performed  [243].  A  two-­‐fold  excess  of   lithium  beyond  the  capacity  of  the  lithiated  positive  electrode  was  used  due  to  imperfect  plating  of  Li  upon  cycling.  The  gravimetric  energy  density  was  found  to  be  roughly  a  factor  of  two  higher  than  for  the  Li-­‐O2  cell.  On  a  volumetric  basis  the  projected  Li-­‐O2  cell  has  no  advantage.  The  American  

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battery   company  Polyplus,   however,   predicts   that   it   should  be  possible   to   construct   a  rechargeable   Li-­‐O2   cell   with   an   energy   density   of   700-­‐800   Wh/kg   using   a   protected  lithium  anode  and   claims   to  have   achieved   this   energy  density   already   (for   a  primary  cell)   [244,245].   Moreover,   the   power   capabilities   are   limited   and   one   major   limiting  factor  being  the  diffusion  of  the  O2  molecules  in  the  electrolyte.      The  attractive  gravimetric  energy  density  will  be  further  reduced  in  practical  Li-­‐O2  cells;  by   the   need   of   the   conductive   porous  matrix   into  which   the   Li2O2  must   be   deposited  owing   to   its   very   low   conductivity,   by   the   electrolyte,   by   separators,   by   current  collectors  and  by  the  packaging  required  at  the  system  level,  and  by  pumps  needed  for  the  oxygen  handling  and  supply.  If  air  is  the  source  for  oxygen,  the  air  should  preferably  be  free  of  water,  particles,  CO2,  and  impurities,  which  will  require  further  cleaning  steps  and  adding  to  the  system  complexity.  Furthermore,  if  oxygen  is  used  directly,  an  oxygen  tank,   preferably   compressed   oxygen,   will   add   to   the   system   weight.   Which   air   or   O2  quality  is  needed?  How  large  an  oxygen  tank  is  needed  for  the  driving  range  aimed  at?      Another   challenge   for   vehicle   applications   is   the   large   voltage   hysteresis   and  polarisation   of   about   1   V   between   the   charge   and   the   discharge.   From   a   material  perspective   the   electrolyte   decomposition   and   the   poor   cyclability   and   catalyst  degradation  are  research  challenges  where  the  electrolyte  seems  to  be  the  key.    

2.7 Organic  concepts  As   an   alternative   to   inorganic   electrode  materials   electroactive   organics   or   polymers  with  reversible  redox  reactions  are  promising  candidates  as  electrode  materials  for  the  new   generation   of   “green   batteries”.   This   is   mainly   ascribed   to   higher   theoretical  capacity,   safety,   sustainability,   environmental   friendliness,   and   potential   low   cost  [246,247].  Many  organic  alternatives  indeed  have  several  distinct  merits  over  inorganic  electrode   materials   in   reaction   kinetics,   structure   diversity,   flexibility,   and  processability.   The   large-­‐scale   use   of   transition-­‐metal   based   electrode   materials   is  somewhat   of   an   unsustainable   route   towards   the   devolvement   of   batteries,   mainly  because  of  resource  limitations,  environmental  pollution,  and  large  energy  consumption  in   both   synthesis   and   recycling   [246].   Ideally,   organic   electrode   materials   could   be  extracted  directly  or  synthesised  from  biomass  [248-­‐251].      For  a  long  time,  organic  electrode  materials  have  received  much  less  attention  compared  to   inorganic   electrode   materials   mainly   due   to   their   relatively   poor   electrochemical  performance  and  the  great  success  of  inorganic  electrode  materials  in  both  research  and  application.   During   the   past   decades,   the   research   on   organic   electrode  materials   has  increased   and   a   lot   of   different   organic   structures   and   redox  mechanisms   have   been  investigated.   Nowadays,   the   comprehensive   electrochemical   performance   of   some  organic   cathode   materials,   including   energy   density,   power   capability,   and   cycling  stability,  are  comparable  or  even  superior  to  the  conventional  inorganic  cathodes  [247].  One   of   the   main   drawbacks   is,   however,   the   currently   attainable   volumetric   energy  density.  A  battery  pack  with   the  energy  needed  will   likely  be   larger   than   the  available  space   in   the   vehicle.   Being   a   rather   new   research   field   there   are   some   interesting  concepts,  which   could   reach   the   vehicle   industry   even   if   the   time  horizon   likely   is   far  beyond  2025.  Therefore,  only  a  short  review  of  the  technology  and  few  potential  routes  is  given  here.  

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2.7.1 Basics  For  inorganic  electrode  materials,  the  redox  reactions  are  related  to  the  valence  change  of  the  transition-­‐metal  or  elemental  substance,  while  for  organic  electrode  materials,  the  redox  reactions  are  based  on  the  charge  state  change  of  an  electroactive  organic  group.  Generally,  organic  electrode  materials  can  be  divided  into  three  types  according  to  their  different   redox   reactions:   i)   n-­‐type  organics,   the   reaction   is   between   the  neutral   state  and  the  negatively  charged  state,  ii)  p-­‐type  organics,  the  reaction  is  between  the  neutral  state   and   the   positively   charged   state,   and   iii)   bipolar   organics,   for  which   the   neutral  state   can   be   either   reduced   to   a   negatively   charged   state   or   oxidised   to   a   positively  charged   state.   There   must   of   course   be   a   potential   difference   between   anode   and  cathode  large  enough  for  practical  use  and  the  redox  state  of  the  active  material  in  both  the  cathode  and  anode  must  hence  be  considered.      To  be  of  interest  in  vehicle  applications  the  cell  voltage  should  preferably  be  higher  than  2.5  V  and  the  cell  should  not  only  show  high  energy  density,  but  also  high  rate  capability.  One   strength   of   organic   electrode   materials   is   their   inherent   fast   reaction   kinetics,  compared  to  the  slow  Li-­‐ion  diffusion  kinetics  in  the  bulk  inorganic  particles.  Conducting  polymers,   nitroxyl   radical   polymers,   and   conjugated   carbonyl   compounds   are   all  promising   candidates   for   high   power   electrodes   [252-­‐256].   As   a   first   example  benzoquinone  (redox  potential  of  2.8  V)  has  a  theoretical  energy  density  of  about  1400  Wh/kg   [257]  much   higher   than   that   of   both   commercial   LiCoO2   (ca.   550  Wh/kg)   and  potential  Li-­‐Mn  rich  NMC  (ca.  1000  Wh/kg)  [258].  Nitroxyl  radical  polymers  can  retain  97%  of  the  theoretical  capacity  even  at  a  charge  rate  of  1200  C  and  discharge  rate  of  60  C  [259].  The  electronic  conductivity  of  organic  electrode  materials  is  usually  low,  or  non-­‐existing,  except  for  conducting  polymers.  This  is  a  serious  hinder  for  the  full  utilisation  of   the   high   rate   performance   of   the   active  materials.   Just   like   the   approaches   for   the  LiFePO4   and   S   cathode,   adding   conductive   carbon   could   significantly   improve   the  electron   transport   in   the   electrode   and   ensure   a   high   utilization   ratio   of   the   active  material.  One  approach   to  overcome  this  drawback   is   to  mix   the  active  materials  with  excellent   conductive   carbon,   e.g.   graphene   [260].   From   a   durability   perspective  dissolution  of   active  materials   in   the  electrolyte   is   a  major   issue   for  organic   electrode  materials.      

2.8 Asymmetrical  super  capacitors  Super   capacitors   have   been   plentifully   investigated   for   hybrid   electric   vehicles,  especially   for  heavy-­‐duty  applications.  The  high  power  density  and   long  durability  are  the  main  advantages.  The  drawbacks  are  mainly  the  low  energy  density  and  thereby  the  high   cost   of   a   pack.   Besides   the   ‘traditional’   capacitance,   a   capacitor   can   be   made   of  pseudo-­‐capacitance  character;  asymmetrical  super  capacitors.  The  energy  is  achieved  by  redox  reactions,  electrosorbtion  on  the  surface  of  the  electrode  by  specifically  absorbed  ions,  and  result  in  a  reversible  faradic  charge-­‐transfer  of  the  electrode.  Depending  on  the  materials   chosen   for   electrodes   and  electrolyte,   different   kinds  of   high-­‐energy  density  capacitors  can  be  tailored  to  suit  a  variety  of  applications  and  needs.      There   are   two   fundamental   ways   to   increase   the   energy   density   of   a   capacitor:   by  increasing  the  cell  voltage  or  the  capacitance.  One  way  to  increase  the  cell  voltage  is  by  changing   the   type   of   electrolyte.   Another   way   is   to   utilise   the   advantages   of  

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asymmetrical  capacitors  employing  both   faradic  and  non-­‐faradic  processes   to   increase  the   capacitance.   Coupling   a   redox  material  with   a  high   capacitance  material   results   in  both  higher  operational  cell  voltage  and  higher  cell  capacitance.  One  attractive   type  of  asymmetrical  super  capacitors  is  made  by  using  activated  carbon  as  one  electrode  and  an  insertion  electrode  of  a  Li-­‐ion  cell  as  the  other,  so  called  Li-­‐ion  capacitors.  The  high  operational   voltage   enables   Li-­‐ion   capacitors   of   both   high   power   and   high   energy  density:  ca.  5  kW/kg  and  20-­‐30  Wh/kg  is  currently  possible.      Some   companies   have   already   commercialised   Li-­‐ion   capacitors,   e.g.   JM   Energy,   FDK,  ATC.  Another  commercial  example  of  this  type  of  capacitors  is  developed  by  Fuji  Heavy  Industry,  using  a  pre-­‐lithiated  carbon  anode  together  with  an  activated  carbon  cathode,  resulting  in  a  cell  of  3.8  V  and  an  energy  density  of  more  than  15  Wh/kg  [261,262].  The  drawback   is  a   limiting  charging  rate  and   the   low-­‐temperature  performance  due   to   the  graphite  based  insertion  anode.  Moreover,  the  process  of  pre-­‐lithiation  of  the  anode  may  lead   to   poor   cost-­‐effectiveness   or   instability   of   the   quality   in   mass   production.  Therefore,  research  is  ongoing  to  find  alternative  solutions  mainly  for  the  anode.    

2.8.1 Research  trends  In  most   cases,   the   faradic   electrodes   lead   to   an   increase   in   the   energy   density   at   the  expense  of  cyclability   (for  balanced  positive  and  negative  electrode  capacities).  This   is  certainly   the  main   drawback   of   asymmetrical   capacitors   and   it   is   important   to   avoid  transforming  a  good  super  capacitor  into  a  mediocre  battery  [261].    The   main   activities   on   asymmetric   super   capacitors   arise,   however,   from   the   Li-­‐ion  batteries   research   field.   At   first,   a   nano-­‐structured   anode   of   Li4Ti5O12   was   combined  with  an  activated   carbon   cathode,   resulting   in   a  2.8  V   cell   exceeding  10  Wh/kg   [263].  The  Li4Ti5O12  anode  ensured  high  power  capacity,  as  well  as  long-­‐life  cyclability  thanks  to  low  volume  change  during  cycling.  Following  this  pioneering  work,  many  studies  have  been   conducted   on   various   combinations   of   a   lithium-­‐insertion   electrode   with   a  capacitive  carbon  electrode.  An  advantage  for  these  types  of  materials   is   the  very  high  rate   capabilities.   Laboratory   cells   have   shown   C-­‐rates   of   100-­‐300   [264,265].   At   these  high   C-­‐rates,   the   reversible   capacity   is   about   95%  of   the   capacity   obtained   at   1C   rate  [265].    Generally  metal  oxides  have  been  the  mostly  employed  active  electrode  materials  for  the  super   capacitors   applications   due   to   their   physico-­‐chemical   properties.   Various  metal  oxides,  such  as  RuO2,  MnO2,  V2O5,  Fe3O4  and  α-­‐Co(OH)2  have  been  studied.  MnO2  is  one  of  the  most  studied  materials  as  a  low-­‐cost  alternative  to  RuO2  (see  below).  Its  pseudo-­‐capacitance   arises   from   the  MnIII/MnIV   oxidation   state   change   at   the   surface   of  MnO2  particles   [266].   Nano-­‐powders   and   nano-­‐structures   of   MnO2   can   further   improve   the  capacitance   [267].  Amorphous  nano-­‐structured  MnO2   electrode  has   shown   impressive  stability   results:   up   to   1200   cycles   at   250   F/g   of   capacitance   [268].   Furthermore,   2D  nano-­‐sheets   of   MnO2   have   been   prepared   by   the   exfoliation-­‐reassembling  method,   to  achieve  specific  capacitance  values  of  about  140-­‐160  F/g  and  a  cycling  stability  of  93-­‐99%  up  to  1000  cycles    [269].      Moreover,  different  morphologies  of  amorphous  RuO2  are  promising  electrode  materials  with  excellent  electrochemical  capacitance  behaviour:  nano-­‐needles  [270],  nano-­‐porous  film  [271]  and  nano-­‐particles  [272].  Depending  on  the  synthesis  method  large  variations  

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of   capacitances   can   be   achieved.   For   example   nano-­‐tubular   RuO2   has   shown   high  capacitance  of  about  1300  F/g  [273],  with  another  method  only  390  F/g  [274],  or  with  yet  another  method  as  low  as  50  F/g  [271].      Recently,  binary  oxides  of  nickel  and  cobalt  (Ni-­‐Co)  have  been  investigated.  Nano-­‐sheets  with   mesoporous   structure   were   studied   showing   a   capacitance   up   to   1846  F/g   and  excellent   rate   capability   [275].   Based   on   this  material,   asymmetrical   super   capacitors  were  made  by  using  the  Ni-­‐Co  oxide  as  the  cathode  and  three  kinds  of  activated  carbons  respectively  as  the  anode.  The  operating  voltage  range  was  0–1.6  V  with  a  capacitance  of  202  F/g  and  a  maximum  energy  density  of  71.7  Wh/kg  in  combination  with  a  maximum  power  density  of  16  kW/kg  (at  an  energy  density  of  41.6  Wh/kg).      Composite   cathodes   of   Ni3S2   nano-­‐particles   and   3D   graphene   have   also   been  investigated.  Due  to  a  synergistic  effect,  the  capacitance  and  the  diffusion  coefficient  of  electrolyte   ions   of   the   activated   composite   electrode   are   ca.   4-­‐6   times   higher   than  electrodes   made   of   only   Ni3S2   [276].   The   composite   cathode   showed   a   specific  capacitance   of   3296   F/g.   In   full   cell   experiments,   a   composite   anode   of   Fe3O4   nano-­‐particles  and  chemically   reduced  graphene  oxide  was  used,  and   the  asymmetric   super  capacitor  was  operated  reversibly  between  0  and  1.6  V  and  a  specific  capacitance  of  233  F/g  was  obtained,  which  delivers  a  maximum  energy  density  of  82.5  Wh/kg  at  a  power  density   of   930  W/kg   [276].   Thus,   for   both   the  Ni3S2,   Fe3O4,   and  Ni-­‐Co   oxides   the   cell  voltages  are   too   low  to  be  attractive   for  vehicle  applications.  Therefore,  other   types  of  electrodes  are  needed  to  increase  the  cell  voltage.    Moving   from   aqueous   to   organic   electrolytes   is   crucial   to   enable   an   increased   cell  voltage   from   0.9   V   to   2.5–2.7   V.   As   the   energy   density   is   proportional   to   the   voltage  squared,   numerous   research   efforts   have   been   directed   at   the   design   of   highly  conducting,  stable  electrolytes  with  a  wider  voltage  window.  Today,  the  state  of  the  art  is  the  use  of  organic  electrolytes  based  on  acetonitrile  or  propylene  carbonate,  the  latter  becoming  more  popular,  because  of  the  high  flash  point  and  lower  toxicity  compared  to  acetonitrile.   Ionic   liquids   have   been   shown   to   enable   a   cell   voltage   of   about   4V   and  consequently  the  energy  density  will  increase  by  about  80  %  compared  to  a  3V  cell  if  the  same  capacitance  can  be  achieved.  The  power  capability  of  the  cell  depends  on  the  ionic  conductivity  of  the  electrolyte,  which  affects  the  cell  resistance.  Ionic  liquids  often  have  low   conductivities   at   low   temperatures,  why   elevated   temperatures   often   are  needed.  For  vehicle  applications,   and   in   the   temperature   range  –30   to  +60   °C,  where  batteries  and  super  capacitors  are  mainly  used,   ionic   liquids  still   fail  to  satisfy  the  requirements  because   of   their   low   ionic   conductivity.   However,   the   choice   of   a   huge   variety   of  combinations   of   anions   and   cations   offers   the   potential   for   designing   an   ionic   liquid  electrolyte  with   an   ionic   conductivity   of   40  mS/cm   and   a   voltage  window   of   >4   V   at  room   temperature   [277].  A   careful   choice  of  both   the   anion  and   the   cation  allows   the  design  of  high-­‐voltage  super  capacitors,  and  3  V,  1000  F  commercial  cells  are  available  [278].  The  ionic  conductivity  of  these  liquids  at  room  temperature  is,  however,  low  (few  mS/cm)  and  they  are  therefore  mainly  used  at  higher  temperatures.  One  example  is  CDC  with  an  EMITFSI  ionic  liquid  electrolyte,  which  has  shown  a  capacitance  of  160  F/g  and  ca.   90   F/cm3   at   60   °C   [279].   Moreover,   hybrid   activated   carbon/conducting   polymer  devices   also   show   an   improved   performance  with   cell   voltages   higher   than   3   V   [280-­‐282].   Supported   by   the   efforts   of   the   Li-­‐ion   community   to   design   safer   systems   using  

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ionic   liquids,   the   research  on   ionic   liquids   for   super   capacitors   is   expected   to  have  an  important  role  in  the  improvement  of  capacitor  performance.    

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3 Emerging  battery  technologies  –  Vehicle  implications  Based   on   the   research   trends   and   the   general   research   field   overview   the   vehicle  implications  of  the  emerging  technologies  can  be  treated  in  some  detail  with  respect  to  future  possibilities  and   limitations.  As  previously   indicated   the  energy  density   is  often  available   from   researchers   and   companies   developing   alternative   battery   solutions.  From  a  vehicle  perspective,  the  power  capabilities  are,  however,  equally  interesting,  or  even  more   interesting.  These  data  are  often  not  available.  The  main  reason   is   that   the  electrode  and  cell  designs  highly  affect  the  power  capabilities;  the  materials  as  such  are  not  the  main  source  of  power  as  it  is  for  the  energy.  Therefore,  only  plausible  indications  on   how   different   technologies   will   affect   the   battery   pack   power   when   its   ready   for  vehicle   installation   will   be   given.   Moreover,   likely   cost   trends   for   the   different  technologies  will  be  summarised  based  on  their  basic  layouts  and  materials  used;  exact  costs   can   only   be   given   when   a   mass-­‐production   is   in   place.   The   emerging   battery  technologies   have   been   assigned   levels   of   readiness   in   terms   of   time  needed   to   reach  vehicle  integration.      A   list   of   proposed   actions   for   the   research   and   development   of   materials,   cells,   and  battery   packs   for   the   different   emerging   technologies,   all   aiming   towards   vehicle  applications  and  integration,  is  provided.    

3.1.1 Benchmark  and  measures  In  order  to  set  the  emerging  battery  technologies  in  relation  to  the  state  of  the  art  Li-­‐ion  batteries,   the   following   cell   data   (Table   2)   and   battery   pack   constraints   (Table   3)   are  used.  These  cells  are  available  on  the  market  today  and  used  in  xEV  applications.    Table  2.  Data  for  state  of  the  art  cells.     Brand   Chemistry   Capacity  

(Ah)  Voltage  (V)  

Weight  (kg)  

C-­rate  

Energy  (Wh/kg)  

Energy  (Wh/L)  

HEV   A123   C//LFP   4.4   3.3   0.2   5C-­‐30C  

71   161  

EV   LG  Chem  

C//LMO/NMC   36   3.7   0.9   C/3-­‐2C  

157   275  

 Table  3.  Battery  pack  constraints  for  technology  evaluations.     xEV  type   Nom.  

Voltage  (V)  Current  (A)   Power  (kW)   Total  

energy  (kWh)  

Passenger  car  

HEV   300   200   60   1.5  

  EV   300   200   60   25  Heavy-­duty  bus  

HEV   600   250   150   5  

  EV   600   250   150   100*  *Assuming  100  km  driving  range  and  an  energy  consumption  of  1.0  kWh/km    

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Using  these  cells  to  build  the  battery  packs  according  to  the  specifications  set  in  Table  3,  the  corresponding  battery  pack  constraints  achieved  are  given  in  Table  4.  Moreover,  the  cost  for  the  cells  and  the  corresponding  battery  packs  are  summarised.  The  data  for  cell  cost  are  based  on  Total  Battery  Consulting;  $1200/kWh  for  HEV  cells  and  $220/kWh  for  EV  cells  [1].    Table   4.   Battery   pack   performance   and   cost   of   cells   and   battery   packs.   All   packs   are  designed  to  meet  the  power  requirements  of  Table  3.       xEV  type   No  of  cells   Total  Cell  

weight  (kg)  Total  energy  (kWh)  

Total  cell  cost  ($)  

Passenger  car  

HEV   180   36   2.5   3000  

  EV   240   216   36   7500  Heavy-­duty  bus  

HEV   360   72   5.1   6150  

  EV   640   576   90*   19800  *10%  lower  than  needed.    

3.2 Voltage  The  different  emerging  battery  technologies  exhibit  different  cell  voltages,  both  nominal  and   range.   Furthermore,   some   are   attributed   with   voltage   hysteresis,   which   will   add  constraints  on  the  electrical  system,  and  be  a  source  of  losses.      In   Table   5   the   voltage   ranges   are   given.   Please   note   that   some   emerging   battery  technologies   consist   of   a   wide   range   of   possible  material   combinations,   consequently  resulting  in  wide  variations  of  the  voltage.  Therefore,  some  rough  estimates  have  been  made  in  the  Table  5.    Table  5.  Voltage  ranges  of  the  emerging  battery  technologies  and  corresponding  number  of  cells  for  a  300  V  battery  pack.     Nominal  

voltage  (V)  Voltage  range  

(V)  No.  of  cells  for  ca.  300  V  pack  

Next  gen  Li-­ion   4.6   2.5-­‐5   65  Solid  state  Li   4.6*   2.5-­‐5   65  Na-­ion   3.7   2-­‐4.4   80  Mg   2   0-­‐2   150  Li-­S   2   1.7-­‐2.8   150  Li-­O2   2.5   2.5-­‐2.8  cha,    

3.5-­‐4.5  dch  120  

Organic   2.8   ca.  2-­‐3   105  Asym.  S.C.   3.8   0-­‐3.8   80  *Assuming  NMC  cathode      The  large  variation  of  the  nominal  voltage,  as  well  as  the  voltage  profiles,  will  affect  the  battery  pack  cost  due  to  the  number  of  cells  needed  to  sustain  the  vehicle  system  voltage  level.  

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3.3 Energy  and  Power  density  from  cell  to  pack/system  The   different   material   combinations   will   give   rise   to   different   energy   densities,   both  gravimetric  and  volumetric.  The  electrode  and  cell  designs  are,  however,  the  main  driver  for  energy  or  power  optimised  cells.  Depending  on  cell  format  (cylindrical,  prismatic,  or  pouch)   the   same   materials   can   give   rise   to   large   variations   in   these   performance  parameters.  The  energy  and  power  densities  will  therefore  also  vary  widely  among  the  different  cell  suppliers.  In  Table  6,  new  and  ‘old’  Li-­‐based  technologies  are  summarised  and  the  corresponding  energy  densities  compared  using  only  the  material  properties  as  input.   The   cells   are   intended   to   perform   the   same   amount   of  work   and   it   is   the   total  energy  that  is  referred  to,  not  the  available.        Table  6.  Gravimetric  and  volumetric  energy  densities  for  different  Li-­‐based  technologies.  Only   cell  materials   taken   into   account   in   the   cell   data.   The   calculations   are   based   on  [283].       Wh/kg  

(cell)  Wh/L  (cell)  

Wh/kg  (pack†)  

Wh/L  (pack†)  

Wh/kg  (system†)  

Wh/L  (system†)  

NMC//C   250   520   150   230   150   230  High-­‐energy  NMC//C  

285   575   170   250   170   250  

High-­‐voltage  Li-­‐Ni-­‐rich  NMC//C  

265   550   160   240   160   240  

High-­‐energy  NMC//SiC  

420   850   240   325   240   325  

High-­‐energy  NMC//Li(m)*  

540   1050   290   375   290   375  

Li-­‐S   550   620   300   260   300   260  Li-­‐O2**   770   820   380   320   280***   240***  USABC†   350   750       235   500  †A   battery   pack   of   a   total   energy   content   of   40   kWh   is   assumed.   *50%   excess   Li;  **assuming  Li2O2  as  final  product;  ***25kg  and  30L  has  been  added  for  the  air-­‐handling  system;  †USABC  targets  [284]      For   the   corresponding   pack   data,   the   same  power   capabilities   are   assumed   and   in   all  calculations,   a   40  kWh   (able   to  deliver  80  kW)  battery  pack  has  been   assumed   [283].  The  same  amount  of  pack  components  (housing,  electronics,  fuses  etc.)  beside  the  cells  is  assumed.      As   the   Li-­‐O2   concept   requires   air   or   oxygen   handling   components   extra   weight   and  volume  has  been  included  at  the  system  level.  A  simple  estimate  gives  the  following:  one  cycle  of  40  kWh  will  require  about  300  mole  of  O2.  To  be  able  to  deliver  80  kW,  about  40  g  air/s   is   required.  An  air-­‐handling  system  (compressor,  water  and  CO2  removal  units,  etc.)   from  a  corresponding   fuel  cell   system  has  been  used:  an  air  compressor  of  15  kg  and  12  L  and  an  air-­‐cleaning  system  of  about  10  kg  and  17  L.  An  alternative  would  be  to  use   compressed   clean   oxygen.   In   such   a   case,   the   oxygen   (tank   not   included)   would  

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require  about  2500  L  at  3  bar,  75  L  at  100  bar,  or  10  L  at  700  bar,  even  further  reducing  the   volumetric   energy   density   of   the   Li-­‐O2   technology.   Of   course   also   the  weight   and  volume   of   the   tank   and   supporting   equipment   must   be   included,   again   reducing   the  energy  density.    For   Li-­‐S   batteries,   the   vehicle   installation   pack   would   be   of   the   same   weight   as   an  advanced  Li-­‐ion  pack,  but  about  40%  larger  in  size.            The   energy   density   of   the   non-­‐Li   based   emerging   battery   technologies   offer   other  promising  prospects:   the  Na-­‐ion  cells  currently  offer  about  125  Wh/kg  and  340  Wh/L  [110].  Using  the  same  calculation  basis  as  above,  the  corresponding  battery  pack  would  be  about  60%  heavier,  but  ca.  10%  smaller  than  the  improved  Li-­‐ion  cells  (Table  6).      Today   it   is   too   early   to   compare   the   Mg   battery   technology   with   the   present   Li-­‐ion  technology  in  terms  of  energy  density.  Only  smaller  research  cells  have  been  made  and  most  data  are  valid  for  just  the  cathode  material  as  such.  Yet,  energy  densities  above  300  Wh/g  do  seem  possible  at  cell  level.  This  will,  however,  require  further  research  and  cell  optimisation  efforts.    Organic  battery  technologies  are  a  wide  range  of  concepts,  some  not  at  all  of  interest  for  vehicle  applications.  Some  concepts  have,  however,  high  theoretical  energy  densities  of  for   example   1400  Wh/g   at   the   cell   level.   The   volumetric   energy   densities   have   to   be  further   investigated,  but  are  at  a   first   instance  questionable.  The  question   is  how  or   if  high   gravimetric   and   low  volumetric  density   cells   can  be  used   for   vehicles   and  how  a  battery  pack  can  look  like.    Asymmetrical  super  capacitors  with  high  energy  densities  up  to  the  range  70-­‐80  Wh/kg  at  cell  level  have  been  shown.  Compared  to  Li-­‐ion  batteries  these  energy  levels  are  low,  but  compared   to  conventional   super  capacitors   these  numbers  are  about  one  order  of  magnitude   higher.   Having   an   increased   energy   density   the   power   capabilities   are   not  affected,  resulting  in  an  attractive  candidate  for  high  power  demanding  applications.      The  targets  for  battery  packs  for  battery  electric  vehicles  set  by  USABC  are  350  Wh/kg  and  750  Wh/L  at  cell   level,  and  235  Wh/kg  and  500  Wh/L  on  system  level   [284].   Just  considering  the  gravimetrical  energy  density  the  organic  concepts  may  seem  as  the  most  attractive   alternative,   but  when  both   the  volumetric   and   the   gravimetric  densities   are  considered,   the   main   attractive   technologies   are   Li-­‐based   concepts:   Li-­‐ion   battery  technology   with   Si-­‐based   anodes,   Li-­‐batteries   with   metallic   Li   anode,   Li-­‐S,   and   Li-­‐O2  technologies.   Overall,   high-­‐voltage   cathodes   in   combination   with   a   metallic   lithium  anode  are  the  more  attractive  concepts.      The  power  capabilities  of   the  cells  are   fundamental   for   the  system  design,  and   for   the  fast-­‐charging   possibilities.   The   C-­‐rate   is   the   mainly   used   reference   and   the   power  capabilities   are   related   to   the   capacity   of   the   cells.   Moreover,   if   the   cell   is   optimised  towards   energy   or   power   will   highly   affect   the   power   capabilities.   The   fast   charging  capabilities  will  follow  the  same  trends.    USABC  power  goals  for  different  types  of  vehicles  give  needs  at  pack  level  of  about  2-­‐3  kW/kg  for  an  HEV,  about  1  kW/kg  for  a  PHEV,  and  about  0.2  kW/kg  for  an  EV  [284].  The  

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power  densities  of  the  reference  cells  used  in  this  study  (see  Table  2)  are  in  line  with  the  goals   of   USABC;   ca.   2   kW/kg   for   the   power   optimised   cell   and   ca.   0.3   kW/kg   for   the  energy  optimised  cell.    The  power  capabilities  are  mainly  determined  by  the  electrode  design.  Smaller  particles  with   high   surface   area   and   thinner   electrodes   are   routes   for   increase   the   power  capabilities,  and  at   the  same  time  the  energy  density  will  be  sacrificed.  Therefore,   it   is  difficult  to  put  numbers  on  the  power  densities  and  C-­‐rates.  In  general  terms,  goal  must  be  to  achieve  the  same  or  improved  power  capabilities  as  of  the  present  Li-­‐ion  cells.      Today,  indications  for  the  Na-­‐ion  technology  is  comparable  power  capabilities  as  the  Li-­‐ion   technology.   For   the   other   emerging   technologies,   lower   capabilities   are   to   be  expected   if   the   energy   density   advantages   should   be   able   to   be   utilised.   The  Mg-­‐cells  would  need  a  C-­‐rate  of  ca.  C/5  to  be  attractive  from  an  energy  point  of  view.  Even  lower  C-­‐rates  are  needed   for   the  Li-­‐S  and  Li-­‐O2   technologies,  which  will  have   lower  or  much  lower   power   capabilities   and   C-­‐rates   of   ca.   C/20   are   needed.   As   a   consequence,   fast-­‐charging  is  an  issue/challenge  for  these  technologies.      

3.4 Cost  trends  The   next   generation   Li-­‐ion   battery   cells   are   attributed  with   higher   energy   and   power  densities  per  cost  due  to  cheap  raw  materials.  The  main  trend  towards  lower  cost  for  the  current  Li-­‐ion  technology  is,  however,  driven  by  improved  production  processes,  higher  production  volumes,  standardised  cell  formats  and  balance  of  plant  components.  Hence  cost   reduction   in   general   cannot   be   expected   to   follow   any   linear   trend   starting   from  materials  cost.    Both  Total  Battery  Consultant  and  Avicenne  have  analysed  the  cost  trends  of  improved  Li-­‐ion  cells  and  battery  packs,  summarised  in  Table  7  [1,285].  It  should  be  noted  that  the  trends   presented   are   related   to   cell   and   production   optimisations   of   the   present   cell  technologies  and  materials.  Cost  is  related  to  the  size  and  capacity  of  the  batteries  and  also   the   production   capabilities.   The   reference   studies   conclude   a   cell   cost   of   5  €/[kWh*Ah]  for  pouch  cells  and  50  €/[kWh*Ah]  for  18650  cells  for  2016.      Table  7.  Cost  trends  (€/kWh)  for  energy  optimised  Li-­‐ion  battery  cells  materials,  cells,  and  packs  for  EV  applications  [1,285].     2010   2015   2017   2020   Cost  

reduction  2010-­2020  

Cell  material   180   140   115   115   36%  Cell   315   235   195   175   45%  Battery   pack  w/o  cells  

210   135   60   55   74%  

Total  battery  pack  cost  

525   370   255   230   56%  

   

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As   can   be   noted,   the   largest   cost   reduction   potential   is   related   to   the   battery   pack  components  and  assembly.  This  is  related  to  mass-­‐production  and  modularisation  in  the  assembly.    For   the   cost   estimations   and   comparisons   in   the   present   study   the   following   cell   cost  split  will  be  used  based  on  Avicenne’s  data  [285]:  

-­‐ 65%  of  the  cell  cost  refers  to  material  cost  -­‐ 40%  of  the  material  cost  refers  to  the  cathode,  12%  to  the  anode,  and  10%  to  the  

electrolyte.      Moreover,  75%  of  the  cost  of  a  battery  pack  is  assumed  be  related  to  the  cells.    The  estimated  cost  for  the  emerging  battery  technologies  are  based  on  assessments  of  a  more  complete   field  overview  and  are   summarised   in  Table  7   including   the  main  cost  drivers.   The   cost   estimates   are   related   to   the   improvements   of   the   present   Li-­‐ion  technology  and  refers  to  the  2025  time  line.      Table  7.  Cost  trend  estimates  (cost/storage  capacity)  for  emerging  battery  technologies,  compared  to  the  improved  Li-­‐ion  technology  (-­‐  refers  to  relative  cost  reduction,  +  refers  to  relative  cost  increase).  Technology   Cost  –  cell   Cost  –  pack*   Cost  driver  Solid  Li-­‐metal    

-­‐  8%   -­‐  6%   Anode  cost  1/3  of  Li-­‐ion,  no  Cu  used  

Na-­‐ion   -­‐  13%   -­‐  10%   20%  lower  cell  material  cost  Mg   ±  10%   +  75%   Low  cell  voltage  Li-­‐S   -­‐  40%   >  100%   Low-­‐cost  raw  materials.  High  pack  cost  

due   to   low   cell   voltage   and   poor   rate  capabilities  

Li-­‐O2   ±  0%   +  250%   Low   electrode   cost,   high   electrolyte  cost,   low   cell   voltage   and   poor   rate  capabilities,   extra   components   for  air/oxygen  handling  not  included.  

Organic   -­‐  50%   -­‐  35%   Low  cell  voltage  Asymmetric  super  capacitors**  

±  0%   ±  0%   High   rate   capabilities,   low   energy  density  

*The   same   cost   for   electronics,   control,   and   management   are   assumed   for   all  technologies.  **HEV  application  only.      The  main  drivers   for   cost   improvements  are   thus   related   to   the   cell   capacity  and   rate  capabilities.  Cells  of  the  same  energy  density  and  cell  cost,  but  having  a  cell  voltage  2  V  lower  will  result   in  an  increased  pack  cost  of  about  75%.  The  same  is  true  if   twice  the  number   of   cell   strings   is   needed   to   fulfil   the   power   requirements.   Therefore,   all  emerging  battery   technologies  having  poor   rate   capabilities  must   exhibit   an  extremely  low   cell   cost   to   compensate   for   this.  Not   only   low   cost   of   raw  materials,   but   also   low  production   costs   have   to   be   considered.  Moreover,   cells   of   low   cell   voltage  must   also  exhibit  much  improved  rate  capabilities.  

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3.5 Pro’s  and  con’s  Based  on  the  review  of  the  research  trends  given  in  Chapter  2,  the  following  pro’s  and  con’s  and  special  needs  of  R&D  efforts  are  concluded  (all  in  relation  to  improved  present  Li-­‐ion  technology):    Next  generation  Li-­ion:  +  High  cell  voltage  !  stable  electrolyte  needed.  +  Less  amount  of  Co  used  !  reduced  cost.  Drawback:  less  interesting  for  recycling?  +  Minor  system  changes  -­‐  Small  steps  in  energy  density  at  cell  level    Solid-­state  Li:  +  High  gravimetric  energy  density  +  Very  high  volumetric  energy  density  +   Cost   reduction   mainly   due   to   no   Cu   needed   for   current   collector.   Drawback:   less  interesting  for  recycling?  -­‐   Increased   operational   temperature   needed   (ca.   60-­‐90   °C)   (could   though   be   an  advantage  depending  on  the  overall  vehicle  design)  -­‐  Safety  issues  related  to  metallic  Li    Na-­ion:  +  Abundant  and  less  expensive  raw  materials.  Drawback:  less  interesting  for  recycling?  +  Same  basic  production  tools  and  schemes  as  for  Li-­‐ion  +   Aluminium   current   collectors   for   both   electrodes   (cost   and   weight   reduction).  Drawback:  less  interesting  for  recycling?  -­‐  Slightly  heavier  and  larger  cells  compared  to  Li-­‐ion  of  the  same  capacity    Mg:  +  High  energy  density  due  to  two  electron  redox  reaction  -­‐  Low  cell  voltage  -­‐  Costly  electrolyte?    Li-­S:  +  High  energy  density  at  cell  and  pack  level  +  Cost  reduction:  cheap  cathode  material  -­‐  Low  cell  voltage  -­‐  Poor  rate  capability  -­‐  Production  process  -­‐  Safety  issues  related  to  metallic  Li    Li-­O2:  +  High  energy  density  at  cell  level  -­‐  Low  cell  voltage  and  large  voltage  hysteresis  -­‐  Poor  rate  capability  -­‐  Sensitive  towards  impurities,  water,  CO2,  etc.  due  to  open  system  -­‐  Safety  issues  related  to  metallic  Li  -­‐  Low  energy  density  at  pack/system  level  -­‐  Complex  vehicle  integration    

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Organic:  +  High  gravimetric  energy  density  +  High  rate  capabilities  -­‐  Low  volumetric  energy  density  -­‐  Low  cell  voltage  -­‐  System  design  constraints  unknown    Asymmetric  super  capacitors:  +  High  rate  capabilities  +  High  cell  voltage  +  Concepts  in  production  -­‐  Low  energy  density  compared  to  battery  technologies    For   all   emerging   battery   technologies,   the   durability   is   an   issue   to   be   address   and   is  impossible  to  predict  based  on  cells  at  the  laboratory  scale.    

3.6 Conclusions  and  proposed  actions  The   challenges   facing   all   emerging   battery   technologies   beyond   the   current   Li-­‐ion  technology  are  numerous.  Issues  remain  to  be  solved  on  the  cathode,  the  anode,  and  the  electrolyte  –   i.e.  on  all  parts.  Besides   the   improvements  of   the  Li-­‐ion  technology,   there  are  attractive   solutions   regarding  solid-­‐state  Li   technologies  and   for  Na  and  Mg  based  technologies.  The  drivers  are  mainly  energy  density,  availability  of  raw  materials,  cost,  and   utilisation   of   more   than   one   electron   per   transition   metal.   In   addition   to   these  technical  challenges,   there   is  continuing  uncertainty  about   the  ability  of  Li-­‐O2  and  Li-­‐S  batteries  to  meet  the  volume,  power,  and  cost  goals  of  automotive  batteries.      The  different  emerging  battery  technologies  are  at  various  development  stages  for  being  of  interest  in  vehicle  applications  by  2025.  The  conclusions  from  this  study  are  based  on  careful  assessments  on  a  broad  field  overview  and  can  be  summarised  accordingly:    

o Most  suitable  technologies   for  EV  applications  for  2025  are:  next  generation  Li-­‐ion  and  Li-­‐metal  polymer.    

o Most   suitable   technology   for   HEV   applications   for   2025   is   asymmetric   super  capacitors  having  increased  cell  voltages  

 This  conclusion  takes  the  following  advantages  in  consideration:    EV:  

o High  gravimetric  and  volumetric  energy  densities  o High  cell  voltage,  resulting  in  fewer  cells  needed  and  higher  energy  density  o High  rate  capabilities  o Communalities  with  today’s  cell  production  o Pack  complexity  –  no  extra  components  needed.  Heating  system  though  needed  

for  Li-­‐metal  polymer.  o Cost  reduction  potentials,  both  of  the  cell  materials  and  the  pack  design.  

 HEV:  

o High  cell  voltage,  resulting  in  fewer  cells  needed  and  higher  energy  density  

46  

o High  rate  capabilities  o Communalities  with  today’s  cell  production  o Pack  complexity  –  no  extra  components  needed.    

 The   main   cost   reduction   potential   is,   however,   related   to   the   pack   design   and  components  included  in  the  pack  except  the  cells.    Preferred  post-­‐2025  technologies  are  Na-­‐ion,  Li-­‐S,  and  Mg,  mainly  due  to  cost  reduction  potentials  of   the  cells   (Na-­‐ion  and  Li-­‐S)  and   two-­‐electron  redox  reactions  (Mg),  and   in  additional  all  these  technologies  are  more  long-­‐term  sustainable.    From   this   study   the   following   recommendations   and   actions   are   proposed   for   further  research  and  development  of  these  technologies  for  vehicle  applications:      

o Continue   to   support   efforts   to   stabilise   the   metallic   lithium   surface   during  cycling.  Such  efforts  have  benefits  for  Li-­‐metal,  Li-­‐S  and  Li-­‐O2  technologies.  

o Advance  the  development  of  stable  electrolytes  for  Mg-­‐based  technologies,  as  this  is  the  main  drawback  of  an  attractive  solution  utilising  two  electrons  in  the  redox  reaction,  and  thereby  double  the  capacity.  

o Expand   and   evaluate   options   for   new   Li-­‐S   concepts   that   enable   improved   rate  capability  and  cyclablity.      

o Fundamental   investigations   of   the   reaction   mechanisms   and   dynamics   on   the  air/oxygen  cathode.    

o For  all  emerging  battery  technologies  improved  electrolytes  are  the  key  in  order  to  increase  the  stability  towards  new  cathode  materials,  to  form  stable  electrode  interfaces,  to  be  stabilized  against  reaction  products  in  both  Li-­‐O2  and  Li-­‐S  cells,  and  last  but  not  least  to  improve  safety.  

o Development  of  solid-­‐state  high-­‐voltage  concepts  with  increased  loading  of  active  materials,  utilizing  advanced  solid  electrolytes  with  higher  conductivity,  reducing  the  amount  of  electrolyte  for  increased  energy  density  of  the  entire  battery  

o Continue   the   investigations   of   the   stability   of   interphases   between   all  components  and  materials  of  the  cell,  and  mastering  the  interphase  between  the  electrodes   and   the   electrolyte   through   the  use  of  new  solvents,   salts,   additives,  binders,  etc.  

o At   a   later   stage   –   efforts   on   scaling   up   selected   emerging   battery   technologies  towards  larger  cells  suitable  for  vehicle  applications.    

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